Titanate-based solid electrolyte materials

Abstract

Provided is a titanic acid–based solid electrolyte material that is free from the risk of hydrogen sulfide generation, contains no rare earth elements, and has good electrochemical stability and lithium-ion conductivity.　This titanic acid–based solid electrolyte material 1 is characterized by comprising a titanate that has a structure in which a plurality of host layers 2, formed by the chaining of octahedrons in which six oxygen atoms have been coordinated with titanium atoms by edge sharing in a two-dimensional direction, are layered and lithium ions 3 and positive ions (α) 4 that are at least bivalent are disposed between the host layers 2, a portion of titanium sites in the host layers 2 being replaced with monovalent to trivalent positive ions (β).

Description

[Technical Field]

[0001] This invention relates to titanate-based solid electrolyte materials. [Background technology]

[0002] Lithium-ion rechargeable batteries consist of a positive electrode, a negative electrode, a separator membrane to prevent physical contact between the positive and negative electrodes, and an electrolyte. They charge and discharge by the movement of lithium ions between the positive and negative electrodes through the electrolyte. Because of their excellent energy density and power density, and their effectiveness in miniaturization and weight reduction, lithium-ion rechargeable batteries are used as power sources for laptops, tablet devices, and smartphones. They are also attracting attention as power sources for electric vehicles.

[0003] Conventional electrolytes use liquid electrolytes containing flammable organic solvents, making them prone to leakage. Overcharging and discharging can cause short circuits inside the battery, potentially leading to fire. Therefore, in recent years, research and development has been conducted on all-solid-state lithium-ion secondary batteries that use inorganic solid electrolyte materials instead of liquid electrolytes to improve safety.

[0004] Inorganic solid electrolyte materials used in all-solid-state lithium-ion secondary batteries are classified into two types based on whether the main element forming the backbone is an oxygen atom or a sulfur atom: sulfide-based solid electrolyte materials and oxide-based solid electrolyte materials. Sulfide-based solid electrolyte materials exhibit higher lithium-ion conductivity than oxide-based solid electrolyte materials, but they have high reactivity with water, raising safety issues such as the generation of hydrogen sulfide. Therefore, (La,Li)TiO3 (hereinafter referred to as "LLTO"), Li6La2CaTa2O 12 Li6La2ANb2O 12 (A=Ca, Sr), Li2Nd3TeSbO 12 Methods for improving the lithium ion conductivity of oxide-based solid electrolyte materials such as are being investigated. For example, a method of doping LLTO with 1% to 5% by mass of sulfur has been disclosed (see Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2018-73805 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, the oxide-based solid electrolyte material described in Patent Document 1 contains sulfur, which may lead to the generation of hydrogen sulfide. Furthermore, the use of rare earth elements raises concerns regarding manufacturing costs. Moreover, the demand for lithium-ion secondary batteries equipped with solid electrolytes has been high in recent years, and conventional oxide-based solid electrolyte materials still have the problem of insufficient electrochemical stability.

[0007] The object of the present invention is to provide a titanate-based solid electrolyte material that does not generate hydrogen sulfide, does not contain rare earth elements, and has good electrochemical stability and lithium-ion conductivity, a method for producing the titanate-based solid electrolyte material, and a solid electrolyte and lithium-ion secondary battery using the titanate-based solid electrolyte material. [Means for solving the problem]

[0008] The present invention provides the following titanate-based solid electrolyte material, a method for producing the titanate-based solid electrolyte material, and a solid electrolyte and lithium-ion secondary battery using the titanate-based solid electrolyte material.

[0009] Item 1 A titanate-based solid electrolyte material characterized by comprising a titanate salt, wherein multiple host layers are stacked, each host layer being formed by a chain of octahedra in which six oxygen atoms are coordinated to a titanium atom, with the octahedra being linked together in a two-dimensional direction via edge sharing, and lithium ions and divalent or higher cations (α) are arranged between the host layers, and a portion of the titanium sites in the host layers are substituted with monovalent to trivalent cations (β).

[0010] Item 2. The titanium-based solid electrolyte material according to Item 1, wherein the cation (α) is a divalent to octavalent cation.

[0011] Item 3. The titanium-based solid electrolyte material according to Item 1 or Item 2, wherein the cation (α) is at least one selected from the group consisting of magnesium ion, aluminum ion, calcium ion, zinc ion, strontium ion, barium ion, [Al 13 O4(OH) 24 (H2O) 12 7+ 、[Ga 13 O4(OH) 24 (H2O) 12 7+ and [Zr4(OH)8(H2O) 16 8+ Item 4. The titanium-based solid electrolyte material according to any one of Items 1 to 3, wherein the ionic radius of the cation (α) is 0.50 Å or more.

[0012] Item 5. The titanium-based solid electrolyte material according to any one of Items 1 to 4, wherein the content of the lithium ions present between the layers of the host layer is 35 mol% to 95 mol% with respect to 100 mol% of the ions present between the layers of the host layer.

[0013] Item 6. The titanium-based solid electrolyte material according to any one of Items 1 to 5, wherein the content ratio (cation (α) / lithium ion) of the cation (α) and the lithium ion present between the layers of the host layer is 1 / 99 to 60 / 40 in molar ratio.

[0014] Item 7. The titanium-based solid electrolyte material according to any one of Items 1 to 6, wherein the cation (β) is at least one selected from the group consisting of hydrogen ion, oxonium ion, lithium ion and magnesium ion.

[0015] Item 8. The titanium-based solid electrolyte material according to any one of Items

[0016] ​​​Item 8 A titanate-based solid electrolyte material according to any one of items 1 to 7, wherein more than 0 mol% and 40 mol% or less of the titanium sites in the host layer are substituted with the cation (β).

[0017] Item 9: A titanate-based solid electrolyte material according to any one of Items 1 to 8, wherein the interlayer distance of the host layer is 5 Å to 20 Å.

[0018] A method for producing a titanate-based solid electrolyte material according to any one of items 1 to 9, comprising: (I) reacting titanate having a layered crystalline structure with basic compounds or salts thereof; (II) mixing the compound obtained in step (I) with a salt of cation (α); and (III) mixing the compound obtained in step (II) with a lithium salt.

[0019] Item 11 A method for producing a titanate-based solid electrolyte material according to any one of items 1 to 9, comprising the step (IV) of mixing a layered crystalline titanate, a lithium salt, and a salt of a cation (α).

[0020] Item 12 A solid electrolyte containing a titanate-based solid electrolyte material as described in any one of items 1 to 9.

[0021] Lithium-ion secondary battery having the solid electrolyte described in item 13, item 12. [Effects of the Invention]

[0022] According to the present invention, it is possible to provide a titanate-based solid electrolyte material that does not generate hydrogen sulfide, does not contain rare earth elements, and has good electrochemical stability and lithium-ion conductivity. By using a solid electrolyte having this titanate-based solid electrolyte material, a high-power lithium-ion secondary battery with excellent safety can be obtained. [Brief explanation of the drawing]

[0023] [Figure 1] Figure 1 is a schematic diagram showing a titanate-based solid electrolyte material according to one embodiment of the present invention. [Figure 2] Figure 2 is a schematic cross-sectional view showing a lithium-ion secondary battery according to one embodiment of the present invention. [Figure 3] Figure 3 shows the Nyquist plots of the samples obtained in Examples 1 to 4 and Comparative Examples 1 to 2. [Figure 4] Figure 4 shows the Nyquist plots of the samples obtained in Examples 1 to 4 and Comparative Example 1. [Figure 5] Figure 5 shows the dQ / dV curve of the sample obtained in Example 1. [Figure 6] Figure 6 shows the dQ / dV curve for the sample obtained in Example 2. [Figure 7] Figure 7 shows the dQ / dV curve for the sample obtained in Example 3. [Figure 8] Figure 8 shows the dQ / dV curve for the sample obtained in Example 4. [Figure 9] Figure 9 shows the dQ / dV curve for the sample obtained in Comparative Example 1. [Figure 10] Figure 10 shows the charge-discharge curve of an all-solid-state battery using the sample obtained in Example 1. [Modes for carrying out the invention]

[0024] The following describes an example of a preferred embodiment of the present invention. However, the following embodiments are merely illustrative. The present invention is not limited in any way to the embodiments described below.

[0025] <Titanium-based solid electrolyte materials> The titanic acid-based solid electrolyte material of the present invention is characterized by comprising a lepidocrocite-type titanate, wherein multiple host layers are stacked, each host layer being formed by a two-dimensional chain of octahedra, in which six oxygen atoms are coordinated to a titanium atom, with shared edges. Lithium ions and divalent or higher cations (α) are arranged between the host layers, and some of the titanium sites in the host layers are substituted with monovalent to trivalent cations (β). The lepidocrocite-type titanate may or may not have crystal water between the host layers.

[0026] The host layer is formed by octahedrons, each with six oxygen atoms coordinated to a titanium atom, linked together in a two-dimensional direction via shared edges, forming a single layer that serves as the unit for stacking (layering). While individual host layers are inherently electrically neutral, some of the tetravalent titanium sites are replaced by monovalent to trivalent cations (β) or are vacancies, resulting in a negative charge. The positive charges of lithium ions, cations (α), etc., present between these host layers (hereinafter referred to as "interlayers") compensate for this, thus maintaining the electrical neutrality of this compound.

[0027] More specifically, Figure 1 is a schematic diagram showing a titanate-based solid electrolyte material according to one embodiment of the present invention. As shown in Figure 1, the titanate-based solid electrolyte material 1 has a crystalline structure in which a plurality of host layers 2 are stacked, and lithium ions 3 and cations (α) 4 are arranged between the host layers 2. Furthermore, each host layer 2 is formed by octahedrons in which 6 oxygen atoms are coordinated to titanium atoms, linked together in a two-dimensional direction with shared edges. Note that Figure 1 is a schematic diagram as an example, and the titanate-based solid electrolyte material of the present invention is not limited to the structure shown in the schematic diagram of Figure 1.

[0028] From the viewpoint of further enhancing lithium-ion conductivity, it is preferable that more than 0 mol% and 40 mol% or less of the titanium sites in the host layer are replaced with cations (β), more preferably 5 mol% to 30 mol% of the titanium sites are replaced with cations (β), and even more preferably 10 mol% to 20 mol% of the titanium sites are replaced with cations (β).

[0029] Examples of cations (β) include hydrogen ions, oxonium ions, alkali metal ions, alkaline earth metal ions, zinc ions, nickel ions, copper ions, iron ions, aluminum ions, gallium ions, and manganese ions. From the viewpoint of further enhancing lithium ion conductivity, it is preferable that the cation (β) be at least one selected from the group consisting of hydrogen ions, oxonium ions, lithium ions, and magnesium ions.

[0030] A portion of the titanium sites in the host layer may be vacancies. If vacancies are present, it is preferable that the portion of the titanium sites in the host layer that exceeds 0 mol% and is 20 mol% or less is vacant, from the viewpoint of further improving lithium ion conductivity.

[0031] The titanates that constitute titanate-based solid electrolyte materials have a layered structure in their crystalline structure, and lithium ion conductivity is exhibited by the interlayers acting as two-dimensional lithium ion conduction pathways. Excellent lithium ion conductivity is obtained when cations (α) placed in the interlayers expand the interlayer conduction pathways. Furthermore, it is thought that the host layer and cations (α) act as pillars through strong electrostatic interactions, suppressing changes in the interlayer distance and thereby improving electrochemical stability.

[0032] Therefore, the titanate-based solid electrolyte material of the present invention can enhance both electrochemical stability and lithium-ion conductivity.

[0033] The cation (α) is a cation with a valency of 2 or higher, preferably a cation with a valency of 2 to 8. Specific examples of the cation (α) include monatomic cations such as magnesium ions, aluminum ions, calcium ions, zinc ions, strontium ions, and barium ions; and polycations having a Keggin-type structure ([Al 13 O4(OH) 24 (H2O) 12 ] 7+ , Al 30 O8(OH) 56 (H2O) 24 ] 18+ [Ga 13 O4(OH) 24 (H2O) 12 ] 7+ (etc.), [Zr4(OH)8(H2O) 16 ] 8+ Examples of polynuclear metal cations include magnesium ions, calcium ions, barium ions, and [Al 13 O4(OH) 24 (H2O) 12 ] 7+ [Ga 13 O4(OH) 24 (H2O) 12 ] 7+ Or [Zr4(OH)8(H2O) 16 ] 8+ It is more preferably aluminum ions, barium ions or [Al 13 O4(OH) 24 (H2O) 12 ] 7+ These cations (α) may be used individually or in combination of multiple types. Preferably, the cation (α) is a monatomic cation, from the viewpoint of further improving thermal stability.

[0034] From the viewpoint of further enhancing electrochemical stability, the ionic radius of the cation (α) is preferably 0.50 Å or more, more preferably 0.80 Å or more, even more preferably 1.0 Å or more, and particularly preferably 1.2 Å or more. Furthermore, from the viewpoint of further enhancing lithium ion conductivity, the ionic radius of the cation (α) is preferably 10.0 Å or less, more preferably 5.0 Å or less, and even more preferably 2.0 Å or less.

[0035] In this specification, "ionic radius" can be expressed as the ionic radius determined by Pauling in the case of a monatomic cation. For example, the ionic radius of an aluminum ion is 0.50 Å, that of a lithium ion is 0.60 Å, that of a magnesium ion is 0.65 Å, that of a zinc ion is 0.74 Å, that of a calcium ion is 0.99 Å, that of a strontium ion is 1.13 Å, and that of a barium ion is 1.35 Å. In the case of a multinuclear metal cation, the ionic radius can be expressed as half the interatomic distance obtained from the structure obtained from single-crystal structure analysis, or from small-angle X-ray scattering (SAXS) or wide-field X-ray absorption fine structure (EXAFS). For example, [Al 13 O4(OH) 24 (H2O) 12 ] 7+ Then 4.5 Å, [Zr4(OH)8(H2O) 16 ] 8+ Therefore, it is 2.0 Å.

[0036] In particular, when the cation (α) is a monatomic cation, the ionic radius of the cation (α) is preferably 0.50 Å or more, more preferably 0.80 Å or more, even more preferably 1.0 Å or more, and particularly preferably 1.2 Å or more, from the viewpoint of further enhancing electrochemical stability. Also, when the cation (α) is a monatomic cation, the ionic radius of the cation (α) is preferably 2.5 Å or less, more preferably 2.3 Å or less, and even more preferably 2.0 Å or less, from the viewpoint of further enhancing lithium ion conductivity.

[0037] Furthermore, when the cation (α) is a polynuclear metal cation, the ionic radius of the cation (α) is preferably 2.0 Å or more, more preferably 3.0 Å or more, preferably 10.0 Å or less, and more preferably 5.0 Å or less. In this case, lithium ion conductivity and electrochemical stability can be further enhanced.

[0038] The interlayer distance of the host layer of the titanate constituting the titanate-based solid electrolyte material is preferably 5 Å to 20 Å, and more preferably 8.5 Å to 16 Å. The titanate has a layered structure in its crystal structure, and the interlayers form a two-dimensional lithium ion conduction path, thereby exhibiting lithium ion conductivity. By setting the interlayer distance within the above range, the activation energy for ion conduction is further reduced, and it is believed that lithium ion conductivity is further improved.

[0039] In X-ray diffraction patterns, several equally spaced peaks appearing in the low-angle region (generally 2θ = 20° or less) originate from the layered structure of titanate, and the interlayer distance can be calculated from the diffraction angle (2θ) of the first-order peak appearing at the lowest angle. Specifically, it can be calculated using Bragg's equation "d = nλ / 2sinθ" (where d is the interlayer distance (Å), θ is the diffraction angle (2θ) of the first-order peak divided by 2, λ is the wavelength of the CuKα line, which is 1.5418 Å, and n is a positive integer (n=1 for the first-order peak)).

[0040] Only lithium ions and cations (α) may be arranged between the host layers, or hydrogen ions, oxonium ions, alkali metal ions such as potassium ions and sodium ions may be arranged, as long as they do not impair the desirable physical properties of the present invention.

[0041] From the viewpoint of further enhancing lithium ion conductivity, the lithium ion content between the host layers is preferably 35 mol% to 95 mol%, and more preferably 50 mol% to 95 mol%, based on 100 mol% of the ions present between the host layers. From the viewpoint of further enhancing electrochemical stability, the cation (α) content between the host layers is preferably 0.5 mol% to 50 mol%, and more preferably 2.0 mol% to 30 mol%, based on 100 mol% of the ions present between the host layers. From the viewpoint of further enhancing lithium ion conductivity and electrochemical stability, the content ratio of cation (α) to lithium ions (cation (α) / lithium ion) between the host layers is preferably 1 / 99 to 60 / 40, and more preferably 3 / 97 to 30 / 70 in molar ratio.

[0042] The titanates constituting the titanate-based solid electrolyte material are powder particles of various shapes, including spherical (including those with slight surface irregularities or those with an elliptical cross-section or other shapes that are roughly spherical), columnar (including those with a rod-like, cylindrical, prismatic, rectangular, rectangular, roughly cylindrical, roughly rectangular, or other shapes that are roughly columnar overall), plate-like, block-like, shapes with multiple protrusions (amoeba-like, boomerang-like, cross-shaped, konpeito-like, etc.), and irregular shapes. The particle size is not particularly limited, but the average particle diameter is preferably 0.01 μm to 20 μm, more preferably 0.05 μm to 10 μm, and even more preferably 0.1 μm to 5 μm.

[0043] In this specification, "average particle diameter" means the particle diameter at 50% cumulative volume (50% cumulative volume particle diameter) in the particle size distribution determined by laser diffraction and scattering, i.e., D 50 This refers to the median diameter, and this volume-based cumulative 50% particle size (D 50 The particle size distribution is determined based on volume, and on a cumulative curve where the total volume is set to 100%, the number of particles is counted from smallest to largest, and the particle size at the point where the cumulative value reaches 50% is the particle diameter. These various particle morphologies and particle sizes can be arbitrarily controlled by the shape of the titanate salt used as the raw material, as described later.

[0044] The titanates that make up the titanic acid-based solid electrolyte materials described above include Li 0.14 K 0.05 Al 0.12 Ti 1.73 O 3.7 · 1.0H2O, Li 0.13 K 0.04 Mg 0.16 Ti 1.73 O 3.7 • 1.7H2O and Li 0.39 K 0.09 Ba 0.20 Ti 1.73 O 3.9 • At least one compound selected from the group consisting of 1.0H2O is preferred.

[0045] The titanate-based solid electrolyte material of the present invention exhibits excellent electrochemical stability and lithium-ion conductivity, and since it does not contain sulfur, it can be suitably used as a solid electrolyte material for lithium-ion secondary batteries. Furthermore, because it does not contain sulfur, there is no risk of hydrogen sulfide generation, and because it does not use rare earth elements, it is superior in terms of manufacturing cost.

[0046] (Method for manufacturing titanate-based solid electrolyte materials) The titanic acid-based solid electrolyte material of the present invention is not limited to any particular manufacturing method as long as the above composition can be achieved. For example, a manufacturing method can be described in which a layered crystalline titanic acid (hereinafter referred to as layered titanic acid) is reacted with basic compounds or their salts, a salt of cation (α) and lithium salt, or a lithium salt and a salt of cation (α). Specifically, examples include a first manufacturing method in which layered titanic acid is reacted with basic compounds or their salts, a salt of cation (α) and lithium salt, and a second manufacturing method in which layered titanic acid is reacted with a lithium salt and a salt of cation (α).

[0047] Layered titanic acid is obtained by mixing a lepidocrocite-type titanate (hereinafter referred to as raw material titanate), which has a layered crystalline structure, with an acid (acid treatment), and substituting exchangeable metal cations with hydrogen ions or hydronium ions.

[0048] The acid treatment is preferably carried out under wet conditions. This acid treatment maintains the layered structure of the raw material titanate while replacing cations such as metal ions that are substituting for some of the titanium sites in the host layer, and metal ions between the host layers, with hydrogen ions or hydronium ions, thereby producing layered titanic acid. The term titanic acid here includes hydrated titanic acid, in which water molecules exist between the layers.

[0049] The acid used in the acid treatment is not particularly limited and may be a mineral acid such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, or boric acid, or an organic acid. The acid treatment can be carried out, for example, by mixing the acid with an aqueous slurry of the raw material titanate, with a reaction temperature of 5°C to 80°C and a reaction time of 1 to 3 hours.

[0050] The cation exchange rate can be controlled by appropriately adjusting the type and concentration of the acid and the slurry concentration of the raw material titanate, depending on the type of raw material titanate. Furthermore, from the viewpoint of the interlayer distance of the resulting lepidocrocite-type titanate, it is preferable that the cation exchange rate be 70% to 100% of the exchangeable cation capacity of the raw material titanate. "Exchangeable cation capacity" refers, for example, to the case where the raw material titanate is of general formula A x M y Ti (2-y) When O4 is represented as such [wherein A is one or more alkali metals excluding Li, M is one or more selected from Li, Mg, Zn, Ga, Ni, Cu, Fe, Al, and Mn, x is a number between 0.5 and 1.0, and y is a number between 0.25 and 1.0], it refers to the value expressed as x + my when the valence of M is m.

[0051] As for the raw material titanate, A x M y Ti (2-y) O4 [wherein A is one or more alkali metals excluding Li, M is one or more selected from Li, Mg, Zn, Ga, Ni, Cu, Fe, Al, Mn, x is 0.5 to 1.0, y is a number from 0.25 to 1.0], A 0.2~0.8Li 0.2~0.4 Ti 1.6~1.8 O 3.7~3.95 〔wherein, A is one or more of alkali metals excluding Li〕, A 0.2~0.8 Mg 0.3~0.5 Ti 1.5~1.7 O 3.7~3.95 〔wherein, A is one or more of alkali metals excluding Li〕, A 0.5~0.7 Li (0.27-x) M y Ti (1.73-z) O 3.85~3.95 〔wherein, A is one or more of alkali metals excluding Li, M is one or more selected from Mg, Zn, Ga, Ni, Cu, Fe, Al, Mn (however, in the case of two or more, combinations of ions with different valences are excluded), when M is a divalent metal, x = 2y / 3, z = y / 3, when M is a trivalent metal, x = y / 3, z = 2y / 3, 0.004 ≦ y ≦ 0.4〕 and the like can be mentioned, preferably A 0.5~0.7 Li 0.27 Ti 1.73 O 3.85~3.95 〔wherein, A is one or more of alkali metals excluding Li〕, and A 0.2~0.7 Mg 0.40 Ti 1.6 O 3.7~3.95 〔wherein, A is one or more of alkali metals excluding Li〕 and is at least one selected from the group consisting of

[0052] A first production method in which a basic compound or its salt, a salt of cation (α), and a lithium salt are allowed to act on layered titanate includes a step (I) of mixing layered titanate and a basic compound or its salt, a step (II) of mixing the compound obtained in step (I) and a salt of cation (α), and a step (III) of mixing the compound obtained in step (II) and a lithium salt.

[0053] In step (I), by mixing layered titanate and a basic compound or its salt, the basic compound or its salt undergoes an ion exchange reaction with hydrogen ions, hydronium ions, etc. between layers, and the interlayer distance expands.

[0054] Step (I) is preferably a wet process and is usually carried out by adding basic compounds or their salts directly, or by diluting basic compounds or their salts with water or an aqueous medium, to a suspension of layered titanic acid dispersed in water or an aqueous medium, and stirring. The reaction temperature is preferably 25°C to 85°C, and the reaction time is preferably 1 hour to 3 hours.

[0055] Basic compounds or their salts are not particularly limited as long as they have an interlayer swelling effect on layered titanic acid and can control the interlayer distance to the desired level. Examples include primary to tertiary organic amines, organic ammonium salts, and organic phosphonium salts. Among these, primary to tertiary organic amines and quaternary organic ammonium salts are preferred.

[0056] Examples of primary organic amines include methylamine, ethylamine, n-propylamine, n-butylamine, pentylamine, hexylamine, octylamine, dodecylamine, 2-ethylhexylamine, 3-methoxypropylamine, 3-ethoxypropylamine, octadecylamine, or salts thereof.

[0057] Examples of secondary organic amines include diethylamine, dipentylamine, dioctylamine, dibenzylamine, di(2-ethylhexyl)amine, di(3-ethoxypropyl)amine, or salts thereof.

[0058] Examples of tertiary organic amines include triethylamine, trioctylamine, tri(2-ethylhexyl)amine, tri(3-ethoxypropyl)amine, dipolyoxyethylenedodecylamine, dimethyldecylamine, or salts thereof.

[0059] Examples of quaternary organic ammonium salts include dodecyltrimethylammonium salt, cetyltrimethylammonium salt, stearyltrimethylammonium salt, benzyltrimethylammonium salt, benzyltributylammonium salt, trimethylphenylammonium salt, dimethyldistearylammonium salt, dimethyldidecylammonium salt, dimethylstearylbenzylammonium salt, dodecylbis(2-hydroxyethyl)methylammonium salt, trioctylmethylammonium salt, and dipolyoxyethylenedodecylmethylammonium salt.

[0060] Examples of organic phosphonium salts include tetrabutylphosphonium salt, hexadecyltributylphosphonium salt, dodecyltributylphosphonium salt, and dodecyltriphenylphosphonium salt.

[0061] The amount of basic compounds or their salts added is preferably 1.0 to 2.5 equivalents, and more preferably 1.1 to 2.0 equivalents, relative to the exchangeable cation capacity of the layered titanic acid. If the amount of basic compounds or their salts added is less than the lower limit, a uniform expansion of the interlayer distance may not be possible, and if the amount of basic compounds or their salts added is greater than the upper limit, it may not be economically viable.

[0062] In step (II), by mixing the compound obtained in step (I) with the salt of cation (α), the salt of cation (α) undergoes an ion exchange reaction with ions of basic compounds in the interlayer, allowing bulky cation (α) to be introduced into the interlayer while maintaining the interlayer distance expanded in step (I).

[0063] Step (II) is preferably a wet process and is usually carried out by adding the compound obtained in step (I) directly, or a suspension of the compound obtained in step (I) dispersed in water or an aqueous medium, and the salt of cation (α) directly, or the salt of cation (α) diluted in water or an aqueous medium, and stirring. The reaction temperature is preferably 25°C to 85°C, and the reaction time is preferably 1 hour to 24 hours.

[0064] The salt of cation (α) used in step (II) can be any salt that can introduce cation (α) into the interlayers of layered titanic acid, preferably aluminum chloride hexahydrate, magnesium chloride hexahydrate, [Al 13 O4(OH) 24 (H2O) 12 ] 7+ That is the case.

[0065] The amount of cation (α) salt mixed in step (II) is preferably 0.001 to 0.20 equivalents, and more preferably 0.02 to 0.15 equivalents, relative to the layered titanic acid obtained in step (I). If the amount of cation (α) salt mixed in step (II) is less than the lower limit, the amount of cation (α) introduced between layers will be small, resulting in insufficient electrostatic interaction with the host layer and potentially low electrochemical stability. If the amount of cation (α) salt mixed in step (II) is greater than the upper limit, the ratio of cation (α) to interlayer ions will be large, potentially reducing lithium ion conductivity.

[0066] In step (III), the compound obtained in step (II) is mixed with the lithium salt, causing the lithium salt to undergo an ion exchange reaction with ions of basic compounds in the interlayer.

[0067] Step (III) is preferably a wet process and is usually carried out by adding a suspension of the compound obtained in step (II) dispersed in water or an aqueous medium to the lithium salt directly or diluted in water or an aqueous medium, and stirring. The reaction temperature is preferably 25°C to 85°C, and the reaction time is preferably 1 to 12 hours. After the reaction, the mixture is dried and the solvent such as water is removed to obtain the titanate that constitutes the titanic acid-based solid electrolyte material of the present invention.

[0068] The lithium salt used in step (III) can be any salt that can introduce lithium ions between the layers of layered titanic acid, and examples include lithium hydroxide monohydrate, lithium carbonate, lithium acetate, lithium citrate, lithium chloride, lithium nitrate, lithium sulfate, lithium phosphate, lithium bromide, lithium iodide, lithium tetraborate, LiPF6, LiBF4, etc., with lithium chloride being preferred.

[0069] The amount of lithium salt mixed in step (III) is preferably 1.0 to 3.0 equivalents, and more preferably 1.0 to 2.5 equivalents, relative to the layered titanate obtained in step (I). If the amount of lithium salt mixed in step (III) is less than the lower limit, it may not be possible to sufficiently replace cations other than the interlayer cation (α) with lithium ions, and if the amount of lithium salt mixed in step (III) is greater than the upper limit, it may not be economically viable.

[0070] A second method for producing layered titanic acid, in which layered titanic acid is reacted with a lithium salt and a salt of cation (α), includes a step (IV) of mixing layered titanic acid, lithium salt, and salt of cation (α).

[0071] In step (IV), the layered titanate, lithium salt, and cation (α) salt are mixed, causing the lithium salt and cation (α) salt to undergo ion exchange reactions with interlayer hydrogen ions, hydronium ions, etc.

[0072] Step (IV) is preferably a wet process and is usually carried out by adding a suspension of layered titanic acid dispersed in water or an aqueous medium to a salt of lithium salt and cation (α) directly, or a salt of lithium salt and cation (α) diluted in water or an aqueous medium, and stirring. The reaction temperature is preferably 25°C to 85°C, and the reaction time is preferably 1 to 12 hours. After the reaction, the mixture is dried and the solvent such as water is removed to obtain the titanate that constitutes the solid electrolyte material of the present invention.

[0073] The cation (α) salt used in step (IV) can be any salt that can introduce cation (α) into the interlayers of the layered titanic acid, and is preferably calcium hydroxide or barium hydroxide octahydrate.

[0074] The lithium salt used in step (IV) can be any salt that can introduce lithium ions between the layers of layered titanic acid, and examples include lithium hydroxide monohydrate, lithium carbonate, lithium acetate, lithium citrate, lithium chloride, lithium nitrate, lithium sulfate, lithium phosphate, lithium bromide, lithium iodide, lithium tetraborate, LiPF6, LiBF4, etc., with lithium hydroxide monohydrate being preferred.

[0075] The amount of lithium salt mixed in step (IV) is preferably 1.0 to 3.0 equivalents, and more preferably 1.0 to 2.5 equivalents, relative to the exchangeable cation capacity of the layered titanate. If the amount of lithium salt mixed in step (IV) is less than the lower limit, it may not be possible to sufficiently replace cations other than interlayer cation (α) with lithium ions, and if the amount of lithium salt mixed in step (IV) is greater than the upper limit, it may not be economically advantageous.

[0076] The amount of cation (α) salt mixed in step (IV) is preferably 0.03 to 0.75 equivalents, and more preferably 0.10 to 0.55 equivalents, relative to the exchangeable cation capacity of the layered titanic acid. If the amount of cation (α) salt mixed in step (IV) is less than the lower limit, the amount of cation (α) introduced between the layers will be small, resulting in insufficient electrostatic interaction with the host layer and potentially low electrochemical stability. If the amount of cation (α) salt mixed in step (IV) is greater than the upper limit, it may not be economically viable.

[0077] <Solid electrolyte> The solid electrolyte of the present invention is a solid electrolyte composed of the titanate-based solid electrolyte material described above, and is a layer that does not contain flammable organic solvents and is capable of conducting lithium ions.

[0078] The proportion of titanate-based solid electrolyte material contained in the solid electrolyte is preferably 10% to 100% by mass, more preferably 50% to 100% by mass, and even more preferably 75% to 100% by mass, based on 100% by mass of the total amount of solid electrolyte. The solid electrolyte may contain a binder for binding the particles of the titanate-based solid electrolyte material, and other solid electrolyte materials other than the titanate-based solid electrolyte material of the present invention, to the extent that they do not hinder the excellent effects of the present invention.

[0079] Other solid electrolyte materials include polymer electrolytes obtained as a mixture of a polymer and a lithium salt. Examples of polymers include poly(meth)acrylic acids such as polyacrylic acid (PAA) and polymethacrylic acid (PMAA); poly(meth)acrylates such as poly-2-hydroxyethyl acrylate and poly-2-hydroxyethyl methacrylate; poly(meth)acrylamides such as polyacrylamide (PAAm) and polymethacrylamide (PMAm); polyethylene oxide (PEO), polycarbonate (PC), polyethylene terephthalate (PET), or copolymers thereof. These may be used individually or in combination. Examples of lithium salts include LiPF6, LiClO4, LiBF4, lithium bis(fluorosulfonyl)imide (LiFSI), or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). These may be used individually or in combination.

[0080] The thickness of the solid electrolyte is preferably 0.1 μm to 1000 μm, and more preferably 0.1 μm to 300 μm.

[0081] Methods for forming a solid electrolyte include, for example, sintering a titanate-based solid electrolyte material or manufacturing a solid electrolyte sheet containing a binder. The binder can be the same material as that used for the polymer electrolyte and the binders used for the positive and negative electrodes described later. It is preferable that the sintering temperature be set lower than the heat treatment temperature used when manufacturing the titanate-based solid electrolyte material, so as not to alter its crystal structure during sintering.

[0082] The solid electrolyte of the present invention exhibits excellent electrochemical stability and lithium-ion conductivity, and since it does not contain sulfur, it can be suitably used as a solid electrolyte for lithium-ion secondary batteries. Furthermore, because it does not contain sulfur, there is no risk of hydrogen sulfide generation, and because it does not use rare earth elements, it is superior in terms of manufacturing cost.

[0083] <Battery> The battery of the present invention is a lithium-ion secondary battery having a positive electrode, a negative electrode, and a solid electrolyte disposed between the positive electrode and the negative electrode, wherein the solid electrolyte is the titanate-based solid electrolyte material of the present invention, i.e., it is an all-solid-state battery.

[0084] More specifically, Figure 2 is a schematic cross-sectional view showing a lithium-ion secondary battery according to one embodiment of the present invention.

[0085] As shown in Figure 2, the lithium-ion secondary battery 10 comprises a solid electrolyte 11, a positive electrode 12, and a negative electrode 13. The solid electrolyte 11 has opposing first main surfaces 11a and second main surfaces 11b. The solid electrolyte 11 is composed of a solid electrolyte containing the titanate-based solid electrolyte material of the present invention. The positive electrode 12 is laminated on the first main surface 11a of the solid electrolyte 11. The negative electrode 13 is laminated on the second main surface 11b of the solid electrolyte 11.

[0086] The method for manufacturing the battery of the present invention is not particularly limited as long as it is a method that can produce the battery described above, and a method similar to that of known battery manufacturing methods can be used. For example, one manufacturing method involves sequentially pressing and stacking a positive electrode, a solid electrolyte, and a negative electrode to create a power generation element, housing this power generation element inside a battery case, and crimping the battery case.

[0087] A general-purpose battery case can be used as the battery case for the battery of the present invention. Examples of battery cases include stainless steel battery cases.

[0088] The battery of the present invention, due to the arrangement of the solid electrolyte of the present invention, is highly safe as it does not generate hydrogen sulfide. Because of its high lithium-ion conductivity, the use of a solid electrolyte allows for a high-output battery. Furthermore, it exhibits high electrochemical stability and excellent reliability. Additionally, the arrangement of the solid electrolyte acts as a separation membrane, eliminating the need for existing separation membranes and enabling the thinning of the battery.

[0089] Hereinafter, each component of the battery of the present invention will be described.

[0090] (Positive electrode) The positive electrode constituting the battery of the present invention has a positive electrode current collector and a positive electrode active material layer.

[0091] Examples of the positive electrode current collector include copper, nickel, stainless steel, iron, titanium, aluminum, aluminum alloy, etc., and aluminum is preferable. The thickness and shape of the positive electrode current collector can be appropriately selected according to the use of the battery, etc. For example, it can have a strip-like planar shape. When it is a strip-like positive electrode current collector, it can have a first surface and a second surface as its back surface. The positive electrode active material layer can be formed on one surface or both surfaces of the positive electrode current collector.

[0092] The positive electrode active material layer is a layer containing a positive electrode active material, and may contain a conductive material and a binder as necessary. The positive electrode active material layer may further contain the titanium-based solid electrolyte material of the present invention. By containing the titanium-based solid electrolyte material of the present invention, a positive electrode active material layer with higher electrochemical stability and lithium ion conductivity can be obtained. The thickness of the positive electrode active material layer is preferably 0.1 μm to 1000 μm.

[0093] The positive electrode active material may be a compound capable of occluding and releasing lithium or lithium ions. For example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMnO2), lithium nickel cobalt aluminate (LiNi 0.8 Co 0.15 Al 0.05 O2, etc.), lithium nickel cobalt manganate (LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2, Li 1+x Ni 1 / 3 Mn 1 / 3 Co 1 / 3O2 (0≦x<0.3, etc.), spinel-type oxides (LiM2O4, M=Mn, V), metallic lithium phosphate (LiMPO4, M=Fe, Mn, Co, Ni), silicate oxides (Li2MSiO4, M=Mn, Fe, Co, Ni), LiNi 0.5 Mn 1.5 Examples include O4 and S8.

[0094] Conductive materials are formulated to enhance current collection performance and reduce contact resistance between the positive electrode active material and the positive electrode current collector. Examples of such materials include carbon-based materials such as vapor-grown carbon fiber (VGCF), coke, carbon black, acetylene black, Ketjenblack, graphite, carbon nanofibers, and carbon nanotubes.

[0095] The binder is formulated to fill the gaps between the dispersed positive electrode active material and to bond the positive electrode active material to the positive electrode current collector. Examples of binders include polysiloxane, polyalkylene glycol, ethyl vinyl alcohol copolymer, carboxymethylcellulose (CMC), hydroxypropylmethylcellulosepropyl (HPMC), cellulose acetate, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), butadiene rubber, styrene-butadiene rubber (SBR), styrene-butadiene-styrene copolymer (SBS), styrene-ethylene-butadiene-styrene copolymer (SEBS), ethylene-propylene rubber, butyl rubber, chloroprene rubber, acrylonitrile-butadiene rubber, acrylic rubber, silicone rubber, fluororubber, and urethane rubber, as well as synthetic rubbers such as polyimide, polyamide, polyamideimide, polyvinyl alcohol, and chlorinated polyethylene (CPE).

[0096] One method for manufacturing a positive electrode involves, for example, suspending a positive electrode active material, a conductive material, and a binder in a solvent to prepare a slurry, and then applying this slurry to one or both sides of a positive electrode current collector. The applied slurry is then dried to obtain a laminate of a positive electrode active material-containing layer and a positive electrode current collector. After that, this laminate is pressed. Another method involves mixing the positive electrode active material, a conductive material, and a binder, forming the resulting mixture into pellets, and then placing these pellets on the positive electrode current collector.

[0097] (Negative electrode) The negative electrode constituting the battery of the present invention comprises a negative electrode current collector and a negative electrode active material layer.

[0098] Examples of materials for the negative electrode current collector include stainless steel, copper, nickel, and carbon, with copper being preferred. The thickness and shape of the negative electrode current collector can be appropriately selected depending on the application of the battery, and for example, it can have a strip-shaped planar form. If it is a strip-shaped current collector, it can have a first surface and a second surface as its back surface. The negative electrode active material layer can be formed on one surface or both surfaces of the negative electrode current collector.

[0099] The negative electrode active material layer is a layer containing the negative electrode active material, and may optionally contain a conductive material and a binder. The negative electrode active material layer may further contain the titanate-based solid electrolyte material of the present invention, and by containing the titanate-based solid electrolyte material of the present invention, the negative electrode active material layer can be made to have even higher lithium ion conductivity. The thickness of the negative electrode active material layer is preferably 0.1 μm to 1000 μm.

[0100] Examples of negative electrode active materials include metal active materials, carbon active materials, lithium metal, oxides, nitrides, or mixtures thereof. Examples of metal active materials include In, Al, Si, Sn, etc. Examples of carbon active materials include mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, soft carbon, etc. Examples of oxides include Li4Ti5O 12Examples include LiCoN and others.

[0101] Conductive materials are incorporated to enhance current collection performance and reduce contact resistance between the negative electrode active material and the negative electrode current collector. Examples of such materials include carbon-based materials such as vapor-grown carbon fiber (VGCF), coke, carbon black, acetylene black, Ketjenblack, graphite, carbon nanofibers, and carbon nanotubes.

[0102] The binder is formulated to fill the gaps between the dispersed negative electrode active material and to bond the negative electrode active material to the negative electrode current collector. Examples of binders include polysiloxane, polyalkylene glycol, polyacrylic acid, carboxymethylcellulose (CMC), hydroxypropylmethylcellulosepropyl (HPMC), cellulose acetate, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), butadiene rubber, styrene-butadiene rubber (SBR), styrene-butadiene-styrene copolymer (SBS), styrene-ethylene-butadiene-styrene copolymer (SEBS), ethylene-propylene rubber, butyl rubber, chloroprene rubber, acrylonitrile-butadiene rubber, acrylic rubber, silicone rubber, fluororubber, and urethane rubber, as well as synthetic rubbers such as polyimide, polyamide, polyamide-imide, polyvinyl alcohol, and chlorinated polyethylene (CPE).

[0103] One method for manufacturing a negative electrode involves, for example, suspending a negative electrode active material, a conductive material, and a binder in a solvent to prepare a slurry, and then applying this slurry to one or both sides of a negative electrode current collector. Next, the applied slurry is dried to obtain a laminate of a negative electrode active material-containing layer and a negative electrode current collector. Subsequently, this laminate is pressed. Another method involves mixing the negative electrode active material, a conductive material, and a binder, forming the resulting mixture into pellets, and then arranging these pellets on the negative electrode current collector. [Examples]

[0104] The present invention will be described in more detail below based on specific examples. The present invention is not limited in any way to the following examples, and can be implemented with appropriate modifications without changing its essence.

[0105] For the titanate raw materials used in the examples and comparative examples, and for the resulting powders, the average particle size was measured using a laser diffraction particle size distribution analyzer (Shimadzu Corporation, SALD-2300), and the interlayer distance was confirmed by analysis using an X-ray diffraction analyzer (Rigaku Corporation, Ultima IV). The compositional formula was confirmed using an ICP-AES analyzer (Agilent Technologies, Agilent 5110 VDV) and a thermogravimetric analyzer (Hitachi High-Tech Science Corporation, NEXTA STA300).

[0106] <Raw material titanates and organic titanates> The raw materials titanates and organic titanates used in the examples and comparative examples are as follows.

[0107] (Raw material: titanate) As a raw material titanate, lithium potassium titanate (K), which has potassium ions in the interlayer and lithium ions in the host layer, is a lepidocrocite type. 0.6 Li 0.27 Ti 1.73 O 3.9 This was used. This lepidocrocite-type lithium potassium titanate had an average particle size of 3 μm, was a white powder consisting of plate-like particles, and had an interlayer distance of 7.8 Å.

[0108] (Organic titanate) 30 g of raw titanate was dispersed in 438 mL of deionized water, and 23.3 g of 95% sulfuric acid was added. After stirring at 25°C for 1 hour, the mixture was separated and washed with water. This procedure was repeated twice to obtain lepidocrocite-type titanate, in which some of the potassium and lithium ions were replaced with hydrogen ions or hydronium ions. This lepidocrocite-type titanate was dispersed in 3.2 L of deionized water, 23.5 g of n-butylamine was added, and after stirring while heating at 80°C for 2 hours, the mixture was filtered to obtain organic lepidocrocite-type titanate (organic titanate).

[0109] (Example 1) 3.0 g of aluminum nitrate notahydrate is dissolved in 38.7 mL of deionized water, and 73.8 g of 0.2 mol / L n-butylamine aqueous solution is added dropwise to the aluminum nitrate aqueous solution. After standing at 60°C, aluminum polynuclear metal cations ([Al 13 O4(OH) 24 (H2O) 12 ] 7+ A compound was synthesized. 5.0 g of organic titanate was added to this, and the mixture was stirred at 80°C for 15 hours. After filtration and thorough washing, a powder was obtained. The obtained powder was dispersed in 49.3 g of a 1.0 mol / L aqueous lithium chloride solution, heated and stirred at 80°C for 3 hours, and then filtered and thoroughly washed to obtain a powdered lepidocrocite-type titanate.

[0110] The average particle size of the obtained lepidocrocite-type titanate was 4 μm, and the interlayer distance was 16 Å.

[0111] (Example 2) 20 g of raw titanate was dispersed in 292 mL of deionized water, and 15.5 g of 95% sulfuric acid was added. After stirring at 25°C for 1 hour, the mixture was separated and washed with water. This procedure was repeated twice to obtain lepidocrocite-type titanate, in which some of the potassium and lithium ions were replaced with hydrogen ions or hydronium ions. This lepidocrocite-type titanate was dispersed in 100 mL of deionized water, and an aqueous solution of 4.4 g of barium hydroxide octahydrate and 4.7 g of lithium hydroxide monohydrate dissolved in 560 mL of deionized water was added to this dispersion. After stirring while heating at 40°C for 3 hours, the mixture was filtered and thoroughly washed, and dried in air at 110°C for 12 hours to obtain powdered lepidocrocite-type titanate.

[0112] The average particle size of the obtained lepidocrocite-type titanate was 3 μm, the interlayer distance was 8.9 Å, and the compositional formula was Li 0.39 K 0.09 Ba 0.20 Ti 1.73 O 3.9 It was 1.0H2O.

[0113] (Example 3) 0.94 g of magnesium chloride hexahydrate was dissolved in 46.2 g of deionized water, 5.0 g of organic titanate was added, and the mixture was heated and stirred for 2 hours. After filtration and thorough washing, a powder was obtained. The obtained powder was dispersed in 49.3 g of 1.0 mol / L lithium chloride aqueous solution, heated and stirred at 80°C for 3 hours, and then filtered and thoroughly washed to obtain a powdered lepidocrocite-type titanate.

[0114] The average particle size of the obtained lepidocrocite-type titanate was 3 μm, the interlayer distance was 10 Å, and the compositional formula was Li 0.13 K 0.04 Mg 0.16 Ti 1.73 O 3.7 The result was 1.7H2O.

[0115] (Example 4) 0.70 g of aluminum chloride hexahydrate was dissolved in 28.8 mL of deionized water, 5.0 g of organic titanate was added, and the mixture was heated and stirred for 2 hours. After filtration and thorough washing, a powder was obtained. The obtained powder was dispersed in 49.3 g of 1.0 mol / L lithium chloride aqueous solution, heated and stirred at 80°C for 3 hours, and then filtered and thoroughly washed to obtain a powdered lepidocrocite-type titanate.

[0116] The average particle size of the obtained lepidocrocite-type titanate was 3 μm, the interlayer distance was 9.2 Å, and the compositional formula was Li 0.14 K 0.05 Al 0.12 Ti 1.73 O 3.7 It was 1.0H2O.

[0117] (Comparative Example 1) 65 g of raw titanate was dispersed in 1 L of deionized water, and 50.4 g of 95% sulfuric acid was added. After stirring at 25°C for 1 hour, the mixture was separated and washed with water. This procedure was repeated twice to obtain lepidocrocite-type titanate, in which some of the potassium and lithium ions were replaced with hydrogen ions or hydronium ions. 50 g of this lepidocrocite-type titanate was dispersed in 200 mL of deionized water, and while heating to 70°C and stirring, 324 mL of a 10% aqueous solution of lithium hydroxide monohydrate was added. After continuing to stir at 70°C for 3 hours, the mixture was filtered and removed. After thorough washing with 70°C warm water, the mixture was dried in air at 110°C for 12 hours to obtain powdered lepidocrocite-type titanate.

[0118] The average particle size of the obtained lepidocrocite-type titanate was 3 μm, the interlayer distance was 8.4 Å, and the compositional formula was K 0.07 Li 1.0 Ti 1.73 The result was O4·0.97H2O.

[0119] (Comparative Example 2) Li manufactured by Toyoshima Seisakusho Co., Ltd. 0.33 La 0.55 TiO3(cubic)(LLTO) was used as a comparative example. The average particle size was 5 μm.

[0120] <Impedance Measurement> 0.050 g of the samples of the repidocrosite-type titanates obtained in Examples 1 to 4 and Comparative Example 1 and LLTO of Comparative Example 2 were each placed in a Teflon (registered trademark) container having copper electrodes with a diameter of 0.8 cm at both ends, and pressure was applied so that the thickness of the sample became 1.0 mm, and measurement was performed in the range from 1 MHz to 70 Hz by the AC impedance method (measurement device: COMPACTSTAT manufactured by IVIUM Technologies). Nyquist diagrams are shown in FIGS. 3 and 4. In FIG. 4, the portions of Examples 1 to 4 and Comparative Example 1 in FIG. 3 are shown enlarged. It shows characteristics of a semicircular shape on the high-frequency side and a spike shape on the low-frequency side, and it is considered that the smaller the semicircle on the high-frequency side, the better the ionic conductivity. From FIGS. 3 and 4, it can be seen that in Examples 1 to 4, the ionic conductivity is excellent as much as or more than that of Comparative Examples 1 to 2.

[0121] <dQ / dV Curve Analysis> For the samples of the repidocrosite-type titanates obtained in Examples 1 to 4 and Comparative Example 1, dQ / dV curve analysis was performed. Specifically, a slurry obtained by mixing an evaluation sample, a conductive material, PVdF, and NMP was coated on a copper foil, punched out after drying, a coin cell battery was prepared using lithium metal as the counter electrode, 1.0 M LiPF6 (EC / DMC = 50 / 50 (v / v)) as the electrolyte, and porous PP as the separator, and charge-discharge measurement was performed. A dQdV curve was created from the obtained charge-discharge results.

[0122] The results are shown in FIGS. 5 to 9. Note that FIG. 5 shows the results of the sample of Example 1, FIG. 6 shows the results of the sample of Example 2, FIG. 7 shows the results of the sample of Example 3, FIG. 8 shows the results of the sample of Example 4, and FIG. 9 shows the results of the sample of Comparative Example 1.

[0123] As shown in Figures 5 to 9, the samples from Examples 1 to 4 did not exhibit the large peaks seen in Comparative Example 1 in the dQ / dV curves, confirming their superior electrochemical stability. In particular, the samples from Examples 1 and 2 showed almost no peaks in the dQ / dV curves, confirming their superior electrochemical stability.

[0124] <All-solid-state battery measurement> All-solid-state battery evaluation was performed on the lepidocrocite-type titanate sample obtained in Example 1. Specifically, the sample from Example 1, lithium cobaltate, and a conductive material were mixed in a mass ratio of 8.5:8.5:1. A 10wt% PVdF NMP solution was added to this mixture, and then more NMP was added to form a slurry. This slurry was coated onto aluminum foil and dried at 60°C for 15 hours to create a cathode layer. The viscosity of the slurry was adjusted by mixing a 20wt% binder aqueous solution, prepared by dissolving polyethylene oxide (molecular weight 20000) and lithium bis(trifluoromethanesulfonyl)imide in ultrapure water so that the molar ratio of ethylene oxide moiety to lithium ions was 20:1, with the sample from Example 1 in a mass ratio of 5:3. The slurry was coated onto the dried positive electrode layer, dried at 60°C for 15 hours, and then punched out. A coin cell battery was created using a film made from a solution of polyethylene oxide (molecular weight 400,000) and lithium bis(trifluoromethanesulfonyl)imide dissolved in acetonitrile with a molar ratio of ethylene oxide moiety to lithium ions of 18:1, with lithium metal as the counter electrode and polyethylene oxide (molecular weight 400,000) and lithium bis(trifluoromethanesulfonyl)imide dissolved in acetonitrile as the buffer layer. Charge and discharge measurements were performed at 0.01C and room temperature.

[0125] The results are shown in Figure 10. From Figure 10, it was confirmed that the all-solid-state battery using Example 1 as the solid electrolyte was functioning as a battery. [Explanation of symbols]

[0126] 1…Titanate-based solid electrolyte materials 2…Host layer 3…Lithium-ion 4…Cation (α) 10…Lithium-ion rechargeable battery 11...Solid electrolyte 11a...First main surface 11b...Second main surface 12...Positive electrode 13...Negative electrode

Claims

1. The host layer is constructed of multiple stacked host layers, each consisting of octahedrons, in which six oxygen atoms are coordinated to a titanium atom, linked together in a two-dimensional direction via edge sharing. Lithium ions and divalent or higher cations (α) are arranged between these host layers. A titanic acid-based solid electrolyte material characterized by being composed of a titanate salt in which a portion of the titanium sites in the host layer are replaced with monovalent to trivalent cations (β).

2. The titanate-based solid electrolyte material according to claim 1, wherein the cation (α) is a divalent to octavalent cation.

3. The cation (α) is at least one selected from the group consisting of magnesium ion, aluminum ion, calcium ion, zinc ion, strontium ion, barium ion, [Al １３ O ４ (OH) ２４ (H ２ O) １２ , [Ga ７＋ O １３ (OH) ４ (H ２４ O) ２ , １２ and [Zr ７＋ (OH) ４ (H ８ O) ２ , １６ and the titanium-based solid electrolyte material according to claim 1, which is at least one selected from the group consisting of ８＋ .

4. The titanate-based solid electrolyte material according to claim 1, wherein the ionic radius of the cation (α) is 0.50 Å or more.

5. The titanate-based solid electrolyte material according to claim 1, wherein the lithium ion content present between the layers of the host layer is 35 mol% to 95 mol% relative to 100 mol% of the ions present between the layers of the host layer.

6. The titanate-based solid electrolyte material according to claim 1, wherein the content ratio of the cation (α) to the lithium ion present between the host layers (cation (α) / lithium ion) is 1 / 99 to 60 / 40 in molar ratio.

7. The titanate-based solid electrolyte material according to claim 1, wherein the cation (β) is at least one selected from the group consisting of hydrogen ions, oxonium ions, lithium ions, and magnesium ions.

8. The titanate-based solid electrolyte material according to claim 1, wherein more than 0 mol% and 40 mol% or less of the titanium sites in the host layer are substituted with the cation (β).

9. The titanate-based solid electrolyte material according to claim 1, wherein the interlayer distance of the host layer is 5 Å to 20 Å.

10. A method for producing a titanate-based solid electrolyte material according to any one of claims 1 to 9, comprising: a step (I) of reacting titanate having a layered crystalline structure with basic compounds or salts thereof; a step (II) of mixing the compound obtained in step (I) with a salt of cation (α); and a step (III) of mixing the compound obtained in step (II) with a lithium salt.

11. A method for producing a titanate-based solid electrolyte material according to any one of claims 1 to 9, comprising the step (IV) of mixing a layered crystalline titanate, a lithium salt, and a salt of a cation (α).

12. A solid electrolyte containing the titanate-based solid electrolyte material described in any one of claims 1 to 9.

13. A lithium-ion secondary battery having the solid electrolyte described in claim 12.

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