Solid electrolyte, solid electrolyte sintered body, lithium-ion secondary battery, and method for manufacturing a solid electrolyte

Abstract

To provide a solid electrolyte that exhibits high ionic conductivity and excellent electrical stability when used as a material for a lithium-ion secondary battery, a solid electrolyte sintered body containing the solid electrolyte, and a lithium-ion secondary battery containing the solid electrolyte. [Solution] A solid electrolyte containing lithium, titanium, vanadium, and oxygen, and containing 89.0% or more of a γ-Li3PO4 type structure, or a solid electrolyte containing lithium, titanium, vanadium, and oxygen with a density of 2.170 g / cm³ 3 The above is a solid electrolyte.

Description

[Technical Field]

[0001] The present invention relates to a solid electrolyte, a solid electrolyte sintered body, a lithium-ion secondary battery, and a method for manufacturing a solid electrolyte. [Background technology]

[0002] In recent years, there has been a growing demand for high-output, high-capacity batteries to power devices such as laptops, tablets, mobile phones, smartphones, and electric vehicles (EVs). Among these, all-solid-state lithium-ion batteries, which use solid electrolytes instead of liquid electrolytes such as organic solvents, are attracting attention for their superior charge / discharge efficiency, charging speed, safety, and productivity.

[0003] Lisicon-type materials are attracting attention and being investigated as solid electrolytes. For example, Patent Document 1 describes a solid electrolyte Li containing lithium (Li), silicon (Si), phosphorus (P), and oxygen (O). 3+x Si 1-x P x O4 has been disclosed.

[0004] Furthermore, Non-Patent Document 1 describes a solid electrolyte Li containing Li, vanadium (V), titanium (Ti), and O. 3+x V 1-x Ti x O4 has been disclosed. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-102375 [Non-patent literature]

[0006] [Non-Patent Document 1] ARRodger, J. Kuwano, AR West, Solid State Ionics 15 (1985) 185-198. [Overview of the project] [Problems to be Solved by the Invention]

[0007] Solid electrolytes containing lithium, silicon, phosphorus, and oxygen described in Patent Document 1 and solid electrolytes containing lithium, titanium, vanadium, and oxygen described in Non-Patent Document 1 are known to have lithium ion conductivity. However, in these solid electrolytes, impurity phases are likely to form, and for use as a solid electrolyte in a lithium ion battery that requires high battery performance, the ion conductivity is not sufficient, and the impurity phase may have an adverse effect on electrical stability, leaving room for improvement.

[0008] An object of the present invention is to provide a solid electrolyte having high ion conductivity and excellent electrical stability, a solid electrolyte sintered body containing the solid electrolyte, a lithium ion secondary battery containing the solid electrolyte sintered body, and a method for producing the solid electrolyte when used as a material for a lithium ion secondary battery. [Means for Solving the Problems]

[0009] The configuration of the present invention for achieving the above object is as follows in [1] to

[12] . [1] A solid electrolyte containing lithium, titanium, vanadium, and oxygen, and containing 89.0% or more of a γ-Li3PO4 type structure.

[0010] [2] The solid electrolyte according to [1], having a density of 2.170 g / cm 3 or more.

[0011] [3] A solid electrolyte containing lithium, titanium, vanadium, and oxygen, and having a density of 2.170 g / cm 3 or more.

[0012] [4] The solid electrolyte according to [3], containing 89.0% or more of the γ-Li3PO4 type structure.

[0013] [5] The solid electrolyte according to any one of [1] to [4], represented by the following formula (1). (1) Li A Ti B V C O4 (In the formula (1), 3.40 ≤ A ≤ 3.70, 0.40 ≤ B ≤ 0.60, 0.40 ≤ C ≤ 0.60, and 0.70 ≤ B / C ≤ 1.40.)

[0014] [6] The solid electrolyte according to any one of [1] to [5], wherein the half-value width of the main peak in the powder X-ray diffraction spectrum with CuKα as the radiation source and the diffraction angle 2θ in the range of 21.0 to 21.7° is 0.4° or less.

[0015] [7] The solid electrolyte according to any one of [1] to [6], wherein the bulk ion conductivity of lithium ions at 25 °C is 3.0 × 10 -5 S / cm or more.

[0016] [8] The solid electrolyte according to any one of [1] to [7], in which no redox potential peak is observed in the CV measurement in the range of 0 to 5V.

[0017] [9] A solid electrolyte sintered body containing the solid electrolyte according to any one of [1] to [8].

[0018]

[10] A lithium ion secondary battery containing the solid electrolyte according to any one of [1] to [8].

[0019]

[11] The lithium ion secondary battery according to

[10] , containing the solid electrolyte as a solid electrolyte layer.

[0020]

[12] (1) A step of mixing a compound containing lithium, a compound containing titanium, and a compound containing vanadium, and A method for producing a solid electrolyte, comprising step (2) of calcining the mixture obtained in step (1) in a gas atmosphere with a carbon dioxide concentration of 300 ppm or less. [Effects of the Invention]

[0021] The solid electrolyte in a preferred embodiment of the present invention has high ionic conductivity and excellent electrical stability. Therefore, when used as a solid electrolyte in a lithium-ion secondary battery, for example, a lithium-ion secondary battery with high ionic conductivity and excellent electrical stability can be obtained. [Brief explanation of the drawing]

[0022] [Figure 1] This is the XRD pattern of solid electrolyte 1 prepared in Example 1. [Figure 2] This is the XRD pattern of solid electrolyte 4 prepared in Comparative Example 1. [Modes for carrying out the invention]

[0023] Embodiments of the present invention will be described below. In this specification, a numerical range represented by "~" means a range that includes the numbers written before and after "~" as the lower and upper limits, respectively.

[0024] <Solid electrolyte> The first embodiment of the solid electrolyte according to the present invention (hereinafter also referred to as "this solid electrolyte") is characterized by containing lithium, titanium, vanadium, and oxygen, and containing 89.0% or more of a γ-Li3PO4 type structure, and the second embodiment contains lithium, titanium, vanadium, and oxygen, and has a density of 2.170 g / cm³. 3 The above is the characteristic feature.

[0025] Solid electrolytes are generally composed of a crystalline structure made up of the elements they contain. As in the first embodiment of this solid electrolyte, solid electrolytes containing lithium, titanium, vanadium, and oxygen are known to typically have a crystalline structure containing lithium, titanium, vanadium, and oxygen, represented by the γ-Li3PO4 type structure. When the proportion of the γ-Li3PO4 type structure in the crystalline structure constituting the solid electrolyte is 89.0% or more, the amount of impurity phases in the crystalline structure is low, which allows for high ionic conductivity and improved electrical stability.

[0026] The solid electrolyte is preferably represented by the following formula (1). (1) Li A Ti B V C O4 (In equation (1) above, 3.40 ≤ A ≤ 3.70, 0.40 ≤ B ≤ 0.60, 0.40 ≤ C ≤ 0.60, and 0.70 ≤ B / C ≤ 1.40.)

[0027] In formula (1) above, A represents the lithium content in the solid electrolyte, which is preferably 3.40 or more, more preferably 3.45 or more, and even more preferably 3.48 or more. Furthermore, A is preferably 3.70 or less, more preferably 3.68 or less, and even more preferably 3.66 or less. By having a lithium content within the aforementioned range, a solid electrolyte with excellent lithium ion conductivity can be obtained. Note that the lower and upper limits of A may be any combination.

[0028] In formula (1) above, B represents the titanium content in the solid electrolyte, which is preferably 0.40 or more, more preferably 0.45 or more, and even more preferably 0.48 or more. Furthermore, B is preferably 0.60 or less, more preferably 0.55 or less, and even more preferably 0.52 or less. By having a titanium content within the above range, a solid electrolyte with excellent lithium ion conductivity can be obtained. Note that the lower and upper limits of B may be any combination.

[0029] In formula (1) above, C represents the vanadium content in the solid electrolyte, preferably 0.40 or more, more preferably 0.45 or more, and even more preferably 0.48 or more. Furthermore, C is preferably 0.60 or less, more preferably 0.55 or less, and even more preferably 0.52 or less. By having a vanadium content within the above range, a solid electrolyte with excellent lithium ion conductivity can be obtained. Note that the lower and upper limits of C may be any combination.

[0030] In formula (1) above, the B / C ratio, which represents the titanium to vanadium content ratio in the solid electrolyte, is preferably 0.70 or higher, more preferably 0.75 or higher, and even more preferably 0.80 or higher. Furthermore, the B / C ratio is preferably 1.40 or lower, more preferably 1.30 or lower, and even more preferably 1.25 or lower. By having B / C within the aforementioned range, a solid electrolyte with excellent lithium ion conductivity can be obtained. It can be inferred from the description in Non-Patent Document 1 that the amount of impurity phase consisting of titanium or vanadium is reduced, and lithium ion conductivity is increased. Note that the lower limit and upper limit of B / C may be any combination.

[0031] Since solid electrolytes generally have no overall charge, it is preferable that the relation A×1 + B×4 + C×5 - 2×4 = 0 is satisfied in equation (1). This relation is valid because lithium is a monovalent cation, titanium is a tetravalent cation, vanadium is a pentavalent cation, and oxygen is a divalent anion.

[0032] In equation (1) above, the values ​​of A to C, that is, the ratio of the number of atoms of each constituent element, lithium, titanium, vanadium, and oxygen, that make up the solid electrolyte, can be determined by adding acid to the sample for thermal decomposition, collecting the decomposition product in a fixed volume, and using a high-frequency inductively coupled plasma (ICP) emission spectrometer to determine the content of each element in the solid electrolyte.

[0033] <Crystal structure> • Percentage of γ-Li3PO4 type structures The crystalline structure constituting this solid electrolyte contains a γ-Li3PO4 type structure. In the first embodiment of this solid electrolyte, the proportion of the γ-Li3PO4 type structure is 89.0% or more, preferably 90.0%, more preferably 91.0% or more, and even more preferably 92.0% or more. In the second embodiment of this solid electrolyte, the proportion of the γ-Li3PO4 type structure is preferably 89.0% or more, more preferably 90.0%, even more preferably 91.0% or more, and particularly preferably 92.0% or more. The inclusion of a γ-Li3PO4 type structure in the solid electrolyte is advantageous for lithium ion conduction, and because the γ-Li3PO4 type structure is less reactive with the active material, it has high electrical stability, which is advantageous for battery applications. The upper limit of the proportion of the γ-Li3PO4 type structure in this solid electrolyte is 100%. The proportion (%) of the γ-Li3PO4 type structure in this solid electrolyte was determined by powder X-ray diffraction spectroscopy using Cu-Kα rays as a source, specifically by the method described in the examples.

[0034] • Half-width (FWHM) The crystal structure constituting this solid electrolyte preferably has a full width at half maximum (FWHM) of 0.4° or less for the main peak in the powder X-ray diffraction spectrum with a diffraction angle 2θ in the range of 21.0° to 21.7° using Cu-Kα rays as the source. The main peak of the γ-Li3PO4 type structure is usually in the range of a diffraction angle 2θ of 21.0° to 21.7°. The FWHM is more preferably 0.2° or less, even more preferably 0.150° or less, even more preferably 0.142° or less, and even more preferably 0.130° or less. The fact that the FWHM of the main peak in the range of a diffraction angle 2θ of 21.0° to 21.7° is below the upper limit indicates that the γ-Li3PO4 type structure has high crystallinity (regular arrangement) and excellent lithium ion conductivity. The lower limit of the FWHM is not particularly limited, but may be 0.100° or more.

[0035] <density> In the second embodiment of this solid electrolyte, the density is 2.170 g / cm³. 3 Preferably, 2,200 g / cm³3 More preferably 2.280 g / cm³ 3 That concludes the explanation. Furthermore, in the first embodiment of this solid electrolyte, the density is preferably 2.170 g / cm³. 3 More preferably 2,200 g / cm³ 3 More preferably, 2,280 g / cm³ 3 That's all. By having a density within the aforementioned range, a dense sintered body with few voids can be created, ensuring sufficient lithium ion conduction pathways, thus tending to increase lithium ion conductivity. Furthermore, the increased mechanical strength contributes to improved battery durability. While there is no particular upper limit to the density of this solid electrolyte, for example, 2,500 g / cm³ is a possible value. 3 That is the case. The density of this solid electrolyte can be determined by the method described in the examples.

[0036] <Relative density> The relative density of this solid electrolyte is preferably 87.0% or higher, more preferably 90.0% or higher, and even more preferably 91.5% or higher. Relative density is the ratio (%) of the measured density value to the theoretical density. Having a relative density within the above range allows for the creation of a dense sintered body with few voids, ensuring sufficient lithium ion conduction pathways, thus tending to increase lithium ion conductivity. Furthermore, it also increases mechanical strength, contributing to improved battery durability. The upper limit of the relative density of this solid electrolyte is 100%. The relative density of this solid electrolyte can be determined by the method described in the examples.

[0037] <Bulk ion conductivity> The bulk ion conductivity of this solid electrolyte is preferably 3.0 × 10⁻⁶. -5 S / cm or more, more preferably 3.2 × 10 -5 S / cm or more, more preferably 3.5 × 10 -5 S / cm or higher, particularly preferably 4.0 -5The bulk ion conductivity is S / cm or higher. When the bulk ion conductivity is above the lower limit, it can be suitably used as a solid electrolyte in lithium-ion secondary batteries. The upper limit of the bulk conductivity of this solid electrolyte is not particularly limited, but is preferably 1.0 × 10⁻⁶. -4 It is S / cm. The bulk ion conductivity of this solid electrolyte refers to the bulk ion conductivity of lithium ions at 25°C and can be determined by the method described in the examples.

[0038] <Oxidation-reduction potential> It is preferable that this solid electrolyte does not have an oxidation-reduction potential in the range of 0 to 5 V. That is, it is preferable that no oxidation-reduction peaks are observed in CV measurements in the range of 0 to 5 V. When the crystalline structure constituting this solid electrolyte contains lithium, oxygen, and by-products consisting of titanium and / or vanadium, oxidation potentials originating from Ti(III) are observed around 2.25 V, oxidation potentials originating from V(III) around 3.75 V, oxidation potentials originating from V(IV) around 4.5 V, reduction potentials originating from V(V) around 3.2 V, reduction potentials originating from V(IV) around 2.3 V, and reduction potentials originating from Ti(IV) around 1.2 V. Therefore, the absence of oxidation-reduction potentials in the range of 0 to 5 V in this solid electrolyte containing lithium, titanium, vanadium, and oxygen indicates excellent electrical stability.

[0039] This solid electrolyte is usually in powder form. When using this electrolyte, it may be used as a powder, or it may be used as a solid electrolyte sintered body or a solid electrolyte molded body before sintering, as described later.

[0040] <Method for producing solid electrolytes> The method for producing this solid electrolyte is described below, but is not limited to the method described below as long as a solid electrolyte with the above-mentioned configuration can be obtained.

[0041] The method for producing this solid electrolyte includes a step (1) of mixing a compound containing lithium, a compound containing titanium, and a compound containing vanadium, and a step (2) of calcining the mixture obtained in step (1) under a gas atmosphere with a carbon dioxide concentration of 300 ppm or less. Solid-phase reactions, liquid-phase reactions, etc., can be used in the synthesis of this solid electrolyte. The manufacturing method by solid-phase reaction will be described in detail below.

[0042] (Process (1): Mixing process) In step (1), a compound containing lithium, a compound containing titanium, and a compound containing vanadium are mixed to obtain a mixture containing these compounds. Note that the compound containing lithium and the compound containing titanium may be compounds containing both lithium and titanium, the compound containing lithium and the compound containing vanadium may be compounds containing both lithium and vanadium, the compound containing titanium and the compound containing vanadium may be compounds containing both titanium and vanadium, and the compound containing lithium, the compound containing titanium and the compound containing vanadium may be compounds containing lithium, titanium and vanadium. If step (1) is performed multiple times, a mixing step may be included in which only a portion of the above compound is present. Also, if the mixing step is performed multiple times, the second and subsequent mixing steps may be performed after the calcined product following the calcination step (step (2)) described later, or after any crushing or pulverizing step. In other words, the remaining compound may be mixed with the mixture after the mixing step in which only a portion of the above compound is present, with the calcined product following step (2) described later, or with the powder after any crushing or pulverizing step.

[0043] While there are no particular limitations on the lithium-containing compounds, inorganic compounds are preferred for ease of handling. Examples of lithium compounds include lithium hydroxide monohydrate (LiOH·H2O), lithium carbonate (Li2CO3), and lithium oxide (Li2O). From the standpoint of ease of handling, lithium hydroxide monohydrate (LiOH·H2O) is more preferable. These lithium compounds may be used individually or in combination of two or more.

[0044] While there are no particular limitations on the titanium-containing compounds, inorganic compounds are preferred due to their ease of handling. Examples of titanium compounds include titanium(IV) oxide and titanium(IV) oxide monohydrate. Titanium(IV) oxide is more preferable due to its ease of handling. These titanium compounds may be used individually or in combination of two or more.

[0045] While there are no particular limitations on the vanadium-containing compounds, inorganic compounds are preferred due to their ease of handling. Examples of vanadium compounds include vanadium(V) oxide and ammonium vanadate. Vanadium(V) oxide is more preferable due to its ease of handling. These vanadium compounds may be used individually or in combination of two or more.

[0046] The above-mentioned methods for mixing the raw materials include using a roll mill, ball mill, small-diameter ball mill (bead mill), pot mill, media stirring mill, air jet mill, mortar and pestle, automatic mixing mortar and pestle, pulverizer, or jet mill. For simplicity, the ratio of the raw materials to be mixed is determined by stoichiometric ratios to achieve the composition shown in formula (1) above.

[0047] From the description in Non-Patent Document 1, it can be inferred that when B / C is adjusted to fall within the range detailed in equation (1) above, the amount of impurity phase consisting of titanium or vanadium decreases, and the lithium ion conductivity increases. Furthermore, since lithium atoms tend to leach out of the system during the calcination process described later, the lithium-containing compound mentioned above may be adjusted by adding it in excess of about 10-20% from the range detailed in formula (1) above.

[0048] The mixing process may be carried out under an atmospheric environment. A gas atmosphere of nitrogen gas and / or argon gas is more preferable.

[0049] (drying process) The resulting mixture may be dried as needed. Known drying methods can be used, including heat drying, vacuum drying, hot air drying, fluidized bed drying, and freeze-drying. After drying, if necessary, it may be crushed and pulverized using the method described in the crushing and pulverization process described later.

[0050] (Process (2): Firing process) In step (2), the mixture obtained in step (1) is calcined under a predetermined gas atmosphere. Step (2) is carried out in a gas atmosphere with a carbon dioxide concentration of 300 ppm or less, preferably 100 ppm or less, more preferably 50 ppm or less, even more preferably 10 ppm or less, particularly preferably 1 ppm or less, and most preferably 0.5 ppm or less. By carrying out the process in a gas atmosphere with a carbon dioxide concentration within the above range, the generation of impurities from side reactions between carbon dioxide and heat is suppressed, and a solid electrolyte with a high proportion of γ-Li3PO4 type structures and high density can be easily produced. The gas used can be any known form of gas with adjusted carbon dioxide concentration, such as pure air (a mixture of O2:N2=20:80) or dry air (compressed air), and may also be an inert gas such as nitrogen gas or argon gas.

[0051] The firing temperature is preferably 700°C or higher, more preferably 750°C or higher, even more preferably 800°C or higher, preferably 1200°C or lower, more preferably 1100°C or lower, and even more preferably 900°C or lower. Firing at 700°C or higher allows for sufficient solid solution of titanium into lithium vanadate, improving ionic conductivity, while firing at 1200°C or lower is preferable as it prevents lithium atoms from easily leaching out of the system.

[0052] The firing time is preferably 1 hour or more, more preferably 2 hours or more, preferably 8 hours or less, and more preferably 6 hours or less. When the firing time is within the above range, the ionic conductivity tends to increase in a balanced manner both within the crystal grains and at the grain boundaries, which is preferable. If the firing time is longer than the above range, lithium atoms tend to leach out of the system. The firing time and temperature are adjusted in conjunction with each other.

[0053] The firing process may be carried out in multiple stages. For example, if a two-stage firing process consisting of low-temperature firing and high-temperature firing is performed, the low-temperature firing may be carried out at 400-800°C for 2-12 hours, and the high-temperature firing may be carried out at the aforementioned firing temperature (e.g., 700-1200°C) for a firing time (e.g., 1-8 hours). If the firing process is carried out in multiple stages, a crushing or pulverizing process using a ball mill or mortar and pestle may be included between firing stages for the purpose of crushing or reducing the particle size of the primary fired material.

[0054] Furthermore, to suppress the residue of by-products, high-temperature firing may be performed twice. In the second high-temperature firing step, the firing temperature is preferably 700°C or higher, more preferably 750°C or higher, even more preferably 800°C or higher, preferably 1200°C or lower, more preferably 1100°C or lower, and even more preferably 1000°C or lower. The firing time is preferably 1 hour or less, more preferably 2 hours or more, preferably 8 hours or less, and even more preferably 6 hours or less. Note that "high-temperature firing twice" includes a step of lowering the temperature to room temperature between the first and second firings.

[0055] If the fired product is left in the atmosphere after firing, it may deteriorate due to moisture absorption or reaction with carbon dioxide. It is preferable to transfer the fired product to a dehumidified inert gas atmosphere and store it when the temperature has dropped below 200°C after firing. This is how the solid electrolyte can be obtained.

[0056] (Crushing and dismantling process) The resulting solid electrolyte may have its particle size adjusted by grinding or crushing. The ground or crushed solid electrolyte is sometimes specifically called a powder. Methods for grinding or crushing include using a roll mill, ball mill, small-diameter ball mill (bead mill), pot mill, media stirring mill, air jet mill, mortar and pestle, automatic mixing mortar and pestle, tank disintegrator, or jet mill.

[0057] <Solid electrolyte sintered body> The solid electrolyte sintered body of the present invention (hereinafter also referred to as "this solid electrolyte sintered body") contains this solid electrolyte, and preferably consists of this solid electrolyte.

[0058] <Method for manufacturing solid electrolyte sintered bodies> The method for manufacturing this solid electrolyte sintered body is described below, but is not limited to the method described below as long as a solid electrolyte sintered body with the above-described configuration can be obtained. The method for manufacturing a solid electrolyte sintered body includes a step (3) of molding the solid electrolyte obtained by the method for manufacturing the solid electrolyte, and a step (4) of sintering the molded body obtained by step (3).

[0059] (Process (3): Molding process) In step (3), the solid electrolyte obtained by the method for producing the solid electrolyte is molded to obtain a molded body. As for the molding method, known powder molding methods can be used. Specifically, these include a method that involves adding a solvent to powder to make a slurry, applying the slurry to a substrate, drying it, and then applying pressure (doctor blade method); a method that involves placing the slurry into an absorbent mold, drying it, and then applying pressure (slip molding method); a method that involves placing the powder into a mold of a predetermined shape and compressing it (mold molding method); an extrusion molding method that involves pushing the slurry out of a die to form the product; a centrifugal force method that involves compressing the powder by centrifugal force to form the product; a rolling molding method that involves supplying the powder to a roll press and rolling it; a cold isostatic pressing method that involves placing the powder into a flexible bag of a predetermined shape, placing it in a pressure medium, and applying isostatic pressure; and a hot isostatic pressing method that involves placing the powder into a container of a predetermined shape, creating a vacuum, and applying isostatic pressure to the container at a high temperature using a pressure medium.

[0060] Examples of mold forming methods include the single-push method, which involves placing powder into a fixed lower punch and a fixed die and applying pressure to the powder with a movable upper punch; the double-push method, which involves placing powder into a fixed die and applying pressure to the powder with a movable lower punch and a movable upper punch; the floating die method, which involves placing powder into a fixed lower punch and a movable die, applying pressure to the powder with a movable upper punch, and moving the movable die when the pressure exceeds a predetermined value so that the fixed lower punch relatively enters the movable die; and the withdrawal method, which involves placing powder into a fixed lower punch and a movable die, applying pressure to the powder with a movable upper punch, and simultaneously moving the movable die so that the fixed lower punch relatively enters the movable die.

[0061] The atmosphere in step (3) may be an atmospheric atmosphere. A nitrogen gas and / or argon gas atmosphere is more preferable.

[0062] (Process (4): Sintering Process) In step (4), the molded body obtained in step (3) is sintered to obtain a solid electrolyte sintered body. Step (4) is preferably carried out in a gas atmosphere with a carbon dioxide concentration of 300 ppm or less, more preferably 100 ppm or less, even more preferably 50 ppm or less, particularly preferably 10 ppm or less, and most preferably 1 ppm or less. By carrying out the process in a gas atmosphere with a carbon dioxide concentration within the above range, the generation of impurities from side reactions between carbon dioxide and heat is suppressed, and a solid electrolyte sintered body with a high proportion of γ-Li3PO4 type structures and high relative density can be easily produced. The gas used can be in a known form such as pure air if a gas with adjusted carbon dioxide concentration is used, or it may be an inert gas such as nitrogen gas or argon gas.

[0063] The sintering temperature is preferably 700°C or higher, more preferably 750°C or higher, even more preferably 800°C or higher, preferably 1200°C or lower, more preferably 1100°C or lower, and even more preferably 900°C or lower. Sintering at 700°C or higher improves ionic conductivity, while sintering at 1200°C or lower is preferable because lithium atoms are less likely to leach out of the system.

[0064] The sintering time is preferably 1 hour or more, more preferably 2 hours or more, preferably 8 hours or less, and more preferably 6 hours or less. A sintering time within this range is preferable because it tends to increase ionic conductivity in a balanced manner both within the crystal grains and at the grain boundaries. If the sintering time is longer than this range, lithium atoms tend to leach out of the system. The sintering time and sintering temperature are adjusted in conjunction with each other.

[0065] The sintering process may be carried out in multiple stages. For example, when performing a two-stage firing process consisting of low-temperature sintering and high-temperature sintering, the low-temperature sintering may be carried out at 400-800°C for 2-12 hours, and the high-temperature sintering may be carried out at the aforementioned sintering temperature (e.g., 700-1200°C) and for the sintering time (e.g., 1-8 hours). If the sintering process is carried out in multiple stages, a crushing / grinding process using a ball mill or mortar and pestle may be included between sintering stages for the purpose of crushing or reducing the particle size of the primary sintered material.

[0066] Furthermore, to suppress the residue of by-products, high-temperature sintering may be performed twice. In the second high-temperature sintering step, the sintering temperature is preferably in the range of 700 to 1200°C, more preferably in the range of 750 to 1100°C, and even more preferably in the range of 800 to 1000°C. The sintering time is preferably 1 to 8 hours, and more preferably 2 to 6 hours.

[0067] Sintered materials obtained after sintering may deteriorate if left exposed to the atmosphere due to moisture absorption or reaction with carbon dioxide. It is preferable to transfer the sintered materials obtained after sintering to a dehumidified inert gas atmosphere and store them when the temperature has dropped below 200°C. In this way, the solid electrolyte sintered body can be obtained.

[0068] <Lithium-ion rechargeable battery> One preferred embodiment of the solid electrolyte is its use in a lithium secondary battery. A lithium-ion secondary battery according to one embodiment of the present invention includes the solid electrolyte, preferably as a solid electrolyte layer. The structure of the lithium secondary battery is not particularly limited, but for example, in the case of a lithium secondary battery having a solid electrolyte layer, the structure is formed by stacking a positive electrode current collector, a positive electrode layer, the solid electrolyte layer, a negative electrode layer, and a negative electrode current collector in this order.

[0069] The positive electrode current collector and the negative electrode current collector are not particularly limited as long as their material conducts electrons without undergoing electrochemical reactions. Examples include elements and alloys of metals such as copper, aluminum, and iron, or conductive metal oxides such as antimond-doped tin oxide (ATO) and tin-doped indium oxide (ITO). It is also possible to use current collectors with a conductive adhesive layer on the surface of these conductors. The conductive adhesive layer may consist of granular conductive material or fibrous conductive material.

[0070] Active materials for the negative electrode layer include lithium alloys, metal oxides, graphite, hard carbon, soft carbon, silicon, silicon alloys, and silicon oxide (SiO₂).n (0 < n ≤ 2), at least one selected from the group consisting of a silicon / carbon composite material, a composite material in which silicon is encapsulated in the pores of porous carbon, lithium titanate, and graphite coated with lithium titanate can be mentioned.

[0071] A silicon / carbon composite material or a composite material in which a silicon domain is encapsulated in the pores of porous carbon is preferable because it has a high specific capacity and can increase the energy density and battery capacity. More preferably, it is a composite material in which a silicon domain is encapsulated in the pores of porous carbon, which is excellent in relaxing the volume expansion associated with the lithium absorption / desorption of silicon, and can maintain a good balance of macro-conductivity, micro-conductivity, and ion conductivity in the composite electrode material or electrode layer. Particularly preferably, it is a composite material in which a silicon domain is encapsulated in the pores of porous carbon, the silicon domain is amorphous, the size of the silicon domain is 10 nm or less, and pores derived from porous carbon are present in the vicinity of the silicon domain.

[0072] As the active material for the positive electrode, at least one selected from the group consisting of lithium cobalt oxide, lithium nickel cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel manganese oxide, lithium manganese oxide, lithium manganese-based spinel, lithium manganese nickel oxide, lithium manganese aluminum oxide, lithium manganese magnesium oxide, lithium manganese cobalt oxide, lithium manganese iron oxide, lithium manganese zinc oxide, lithium chromium nickel manganese oxide, lithium chromium manganese oxide, lithium titanate, lithium metal phosphate, transition metal oxide, titanium sulfide, graphite, hard carbon, transition metal-containing lithium nitride, silicon oxide, lithium silicate, lithium metal, lithium alloy, Li-containing solid solution, and lithium storage intermetallic compound can be mentioned.

[0073] Of these, LiNiCoMn oxide, LiNiCo oxide, or LiCo oxide are preferred, with LiNiCoMn oxide being more preferred. This active material has good affinity with solid electrolytes and an excellent balance of macroconductivity, microconductivity, and ionic conductivity. Furthermore, it has a high average potential, which can increase energy density and battery capacity in terms of the balance between specific capacity and stability. In addition, the active material for the positive electrode may be coated on the surface with an ionic conductive oxide such as lithium niobate, lithium phosphate, or lithium borate.

[0074] The active material for the negative electrode layer or positive electrode layer is preferably in particulate form. The 50% diameter in its volume-based particle size distribution is preferably 0.1 μm to 30 μm, more preferably 0.3 μm to 20 μm, even more preferably 0.4 μm to 10 μm, and most preferably 0.5 μm to 3 μm. Furthermore, the ratio of the length of the major axis to the length of the minor axis (length of major axis / length of minor axis), i.e., the aspect ratio, is preferably less than 3, and more preferably less than 2.

[0075] The active material for the negative electrode layer or positive electrode layer may form secondary particles. In that case, the 50% diameter in the number-based particle size distribution of the primary particles is preferably 0.1 μm to 20 μm, more preferably 0.3 μm to 15 μm, even more preferably 0.4 μm to 10 μm, and most preferably 0.5 μm to 2 μm. When forming the electrode layer by compression molding, the active material is preferably primary particles. When the active material is primary particles, it is less likely that the electron conduction path or hole conduction path will be impaired even when compressed.

[0076] (Manufacturing method for lithium-ion secondary batteries) The lithium-ion secondary battery can be obtained by a known powder molding method. For example, by stacking a positive electrode current collector, powder for the positive electrode layer, powder for the solid electrolyte layer, powder for the negative electrode layer, and a negative electrode current collector in this order and simultaneously powder molding them, the formation of the positive electrode layer, solid electrolyte layer, and negative electrode layer, as well as the connections between the positive electrode current collector, positive electrode layer, solid electrolyte layer, negative electrode layer, and negative electrode current collector, can be achieved simultaneously. Alternatively, each layer can be powder molded sequentially. The resulting powder molded product may be subjected to heat treatment such as sintering as needed.

[0077] Powder molding methods include, for example, a method that involves adding a solvent to powder to make a slurry, applying the slurry to a current collector, drying it, and then applying pressure (doctor blade method); a method that involves placing the slurry into an absorbent mold, drying it, and then applying pressure (casting method); a method that involves placing the powder into a mold of a predetermined shape and compressing it (mold molding method); an extrusion molding method that involves pushing the slurry out of a die to form the material; a centrifugal force method that involves compressing the powder using centrifugal force to form the material; a rolling molding method that involves supplying the powder to a roll press and rolling it; a cold isostatic pressing method that involves placing the powder into a flexible bag of a predetermined shape, placing it in a pressure medium, and applying isostatic pressure; and a hot isostatic pressing method that involves placing the powder into a container of a predetermined shape, creating a vacuum, and applying isostatic pressure to the container at a high temperature using a pressure medium.

[0078] Examples of mold forming methods include the single-push method, which involves placing powder into a fixed lower punch and a fixed die and applying pressure to the powder with a movable upper punch; the double-push method, which involves placing powder into a fixed die and applying pressure to the powder with a movable lower punch and a movable upper punch; the floating die method, which involves placing powder into a fixed lower punch and a movable die, applying pressure to the powder with a movable upper punch, and moving the movable die when the pressure exceeds a predetermined value so that the fixed lower punch relatively enters the movable die; and the withdrawal method, which involves placing powder into a fixed lower punch and a movable die, applying pressure to the powder with a movable upper punch, and simultaneously moving the movable die so that the fixed lower punch relatively enters the movable die.

[0079] The thickness of the positive electrode current collector is preferably 1 to 200 μm, more preferably 5 to 150 μm, and even more preferably 10 to 100 μm. The thickness of the positive electrode layer is preferably 1 to 200 μm, more preferably 5 to 150 μm, and even more preferably 10 to 100 μm. The thickness of the solid electrolyte layer is preferably 50 nm to 1000 μm, more preferably 100 nm to 100 μm. The thickness of the negative electrode layer is preferably 1 to 200 μm, more preferably 5 to 150 μm, and even more preferably 10 to 100 μm. The thickness of the negative electrode current collector is preferably 1 to 200 μm, more preferably 5 to 150 μm, and even more preferably 10 to 100 μm. [Examples]

[0080] The present invention will be described in detail below with reference to examples, but the present invention is not limited thereto.

[0081] [Example 1] <Preparation of solid electrolytes> In a 500mL polypropylene bottle, 100g of pure water, 10.00g (0.2383mol) of lithium hydroxide monohydrate (grade: Kagoshima Special Grade, purity: >98.0%, manufactured by Kanto Chemical Co., Ltd.), 2.95g (0.0162mol) of vanadium(V) oxide (grade: Kagoshima Special Grade, purity: >99.0%, manufactured by Kanto Chemical Co., Ltd.), 2.59g (0.0324mol) of titanium dioxide (grade: F-6A, purity: >99%, manufactured by Resonaq Ceramics Co., Ltd.), and 700g of 5mm diameter zirconia balls were placed and mixed using a pot mill at 300rpm for 24 hours under atmospheric conditions. The amount of substance (mol) of each raw material was calculated assuming a purity of 100%. After mixing, the mixture (without the zirconia balls) and a stirring bar were placed in a Teflon evaporating dish and dried using a hot stirrer at 130°C, 70 rpm, for 8 hours. After drying, the collected powder was ground in a mortar. 2g of the crushed mixed powder was placed on an alumina setter and heated to 825°C at a heating rate of 10°C / min in a quartz tube furnace (Asahi Rika Seisakusho Co., Ltd., Ceramic Electric Tubular Furnace ARF-50K) with an outer diameter of 50mm (inner diameter of 46mm) under a pure air atmosphere (carbon dioxide concentration <1ppm), held for 2 hours, and then allowed to cool naturally to obtain calcined powder. The atmosphere during calcination was maintained by flowing pure air into the quartz tube (flow rate 200 sccm). The calcined powder was ground in a mortar to obtain a powder of solid electrolyte 1. Solid electrolyte 1 is calculated from the following formula (2) (Y=Z), Li 3.5 Ti 0.5 V 0.5 It has a composition represented by O4. (2) Li x Ti y V z O4 (In equation (2) above, X = -4Y - 5Z + 8 and Y + Z = 1. In equation (2) above, X represents the lithium content in the solid electrolyte and is determined from the previously determined Y, Z and ion valencies so that the total charge becomes 0.)

[0082] [Example 2] In the preparation of the solid electrolyte, the powder of solid electrolyte 2 was obtained in the same manner as in Example 1, except that the calcination process was carried out under a pure nitrogen (cylinder nitrogen, carbon dioxide concentration <0.3 ppm, flow rate 200 sccm) atmosphere instead of a pure air atmosphere.

[0083] [Comparative Example 1] In the preparation of the solid electrolyte, the firing process was carried out in a dry air atmosphere (carbon dioxide concentration 400-500 ppm, flow rate 200 sccm) instead of a pure air atmosphere, and in the preparation of the sintered solid electrolyte body, the firing process of the molded body was carried out in an atmospheric atmosphere (carbon dioxide concentration 400-500 ppm) instead of a pure air atmosphere. Except for these differences, the powder of solid electrolyte 3 was obtained in the same manner as in Example 1.

[0084] [Comparative Example 2] In the preparation of the solid electrolyte, the firing process was carried out in an atmosphere of air (carbon dioxide concentration of 400-500 ppm) instead of a pure air atmosphere, and in the preparation of the sintered solid electrolyte body, the firing process of the molded body was carried out in an atmosphere of air (carbon dioxide concentration of 400-500 ppm) instead of a pure air atmosphere. In addition, the powder of solid electrolyte 4 was obtained in the same manner as in Example 1.

[0085] [Measurement and evaluation of solid electrolytes and solid electrolyte sintered bodies] The solid electrolytes and solid electrolyte sintered bodies produced in the above examples and comparative examples were measured and evaluated for the following items. These evaluation results are summarized in Table 1 below.

[0086] <Fabrication of solid electrolyte sintered bodies> 0.3 g of the solid electrolyte powder produced in the example or comparative example was placed in a mold (die diameter 10 mm), and the pressure was increased to 200 MPa using a hydraulic press and held for 1 minute to obtain a pellet-shaped molded body. The resulting molded body was placed on an alumina setter and heated to 800°C at a heating rate of 10°C / min in a quartz tube furnace (Asahi Rika Seisakusho Co., Ltd., ceramic electric tubular furnace ARF-50K) with an outer diameter of 50 mm (inner diameter of 46 mm) under the same gas atmosphere used for firing the solid electrolyte. The temperature was maintained for 3 hours, and then allowed to cool naturally to obtain pellet-shaped solid electrolyte sintered bodies with a diameter of φ8-9 mm and a thickness of 1-2 mm. When firing in a pure air atmosphere, this was achieved by flowing gas (flow rate 200 sccm) into the quartz tube.

[0087] <Crystal structure ratio (proportion of γ-Li3PO4 type structure)> Powder X-ray diffraction (XRD) patterns were obtained by grinding the above solid electrolyte sintered body in a mortar and pestle. The powder was then measured using an X-ray diffraction analyzer, X'Pert PRO (PANalytical), with Cu-Kα rays (output: 45kV, 40mA), diffraction angle 2θ: 10~80°, step size: 0.013°, incident side Sollers slit: 0.04rad, incident side Anti-scatter slit: 2°, receiving side Sollers slit: 0.04rad, receiving side Anti-scatter slit: 5mm). The XRD patterns obtained from solid electrolyte 1 and solid electrolyte 4 are shown in Figures 1 and 2, respectively. When measuring solid electrolyte sintered bodies in pellet form, typically only the γ-Li3PO4 type structure contained on the surface of the pellet is observed. However, by using the powder after grinding, the γ-Li3PO4 type structure contained inside the pellet can also be observed, thus allowing for the observation of the γ-Li3PO4 type structure of the entire solid electrolyte. The full width at half maximum (FWHM, in degrees) within the range of the main peak (21.0° ≤ 2θ ≤ 21.7°) of the γ-Li3PO4 type structure obtained from the resulting XRD image was calculated. Furthermore, the proportion (%) of the γ-Li3PO4 type structure was calculated from the sum of the main peak heights (21.0°≦2θ≦21.7°) of the γ-Li3PO4 type structure obtained from the XRD figures and the main peak heights (Li2TiO3: 18°≦2θ≦19°, Li2CO3: 31°≦2θ≦32°, Li2O: 33°≦2θ≦34°, LiTiO2: 43°≦2θ≦44°) of each crystal structure considered to be included in structures other than the γ-Li3PO4 type structure, according to the following formula.

[0088]

number

[0089] The main peaks of each crystal structure were calculated from (measured peak heights of each crystal structure) - (background). Alternatively, this could be confirmed by performing a Rietveld analysis using, for example, the well-known analysis software RIETAN-FP (which can be obtained from the website of creator Fujio Izumi, "RIETAN-FP·VENUS System Distribution File" (http: / / fujioizumi.verse.jp / download / download.html)).

[0090] <Density, relative density> The mass (g) of the solid electrolyte sintered body was measured using an electronic balance. Next, the diameter of the solid electrolyte sintered body was measured using a caliper, and the thickness was measured at four points using a micrometer, and the average value was calculated. From the obtained values, the density of the solid electrolyte (measured value, g / cm³) was calculated according to the following formula. 3 ) was calculated.

[0091]

number

[0092] From the obtained density, the theoretical density of the solid electrolyte (theoretical value, g / cm³) can be calculated according to the following formula. 3 The relative density (%) was calculated as the percentage of the ratio of the measured density to the given density.

[0093]

number

[0094] The theoretical density of the solid electrolyte was calculated by weighting the theoretical densities of each crystal structure constituting the solid electrolyte using the content ratio of each crystal structure, which was determined in the same manner as the method for calculating the crystal structure ratio (or by Rietveld analysis). The values ​​for each crystal structure were taken from publicly known databases (material project). For example, the solid electrolyte (Li) obtained in the examples and comparative examples. 3.5 Ti 0.5 V 0.5 The theoretical density of O4 is 2.485 g / cm³, calculated using the following formula. 3 This was the calculated result.

[0095]

number

[0096] <Bulk ion conductivity> Both sides of the solid electrolyte sintered body (sample) were polished with waterproof sandpaper (#800, #1200), and then finished polished with lapping film sheet (#2000). The sides of the polished solid electrolyte sintered body were covered with masking tape, and gold electrodes were formed on both sides of the solid electrolyte sintered body using a vapor deposition apparatus (QUICK AUTO COATER SC-701AT). After electrode formation, the masking tape on the sides of the solid electrolyte sintered body was removed, and the solid electrolyte sintered body was placed in a 4-terminal sample holder in a constant temperature and humidity chamber (25°C), and impedance measurement was performed using an impedance analyzer SI1260 (Solartron Analytical). The measurement was performed in a frequency range of 10MHz to 0.1Hz and with an applied voltage of 100mV. The bulk ionic conductivity (S / cm) was calculated using the following formula.

[0097]

number

[0098] <Presence or absence of oxidation potential and reduction potential> · Fabrication of a laminate cell for CV measurement The fabrication of the cell for CV measurement was carried out inside a glove box purged with an inert gas. First, both sides of a solid electrolyte sintered body with a diameter of φ8 - 9 mm and a thickness of 1 - 2 mm were polished with waterproof paper (#1200) to make the thickness less than 1 mm. One side was masked, and a working electrode with a diameter of φ6 mm made of Au (gold) was formed on the polished solid electrolyte sintered body using a vapor deposition apparatus (QUICK AUTO COATER SC - 701AT, manufactured by Sunyu Electronics Co., Ltd.). Next, a separator (Celgard 2400, manufactured by Asahi Kasei Corporation) punched out to φ8 mm and wetted with a 1 mol / L LiPF6 electrolyte solution (manufactured by Kishida Chemical Co., Ltd.) in a solvent of ethylene carbonate (EC): diethyl carbonate (DEC) = 1:1 v / v% was placed in contact with and covering the solid electrolyte sintered body. Further, a Li foil (lithium foil, manufactured by Honjo Metal Co., Ltd.) punched out to φ6 mm was placed in contact with and covering the separator to obtain a laminate. In this way, a reference electrode of Li was formed on the solid electrolyte sintered body. The laminate obtained was placed on a 40 mm square insulating aluminum laminate film. Then, nickel tabs were attached to the reference electrode and the working electrode. Finally, a 40 mm square aluminum laminate film was overlapped to cover the laminate, and vacuum lamination was performed so that oxygen did not enter the cell to obtain a cell for CV measurement.

[0099] <CV measurement (presence or absence of oxidation - reduction potential)> · CV measurement For the obtained cell for CV measurement, CV measurement was carried out using BCS - 805 (manufactured by Toyo Technica Co., Ltd.) under the measurement conditions of 1 mV / s, 0 - 5 V, and 25 °C. If oxidation peaks were observed around 2.25V, originating from Ti(III), around 3.75V, and around 4.5V, it was indicated as "(1)". If reduction peaks were observed around 3.2V, originating from V(V), around 2.3V, and around 1.2V, it was indicated as "(2)". If no oxidation-reduction peaks were observed in the range of 0-5V, it was indicated as "None".

[0100] [Table 1]

Claims

1. It contains lithium, titanium, vanadium, and oxygen. γ-Li ３ PO ４ A solid electrolyte containing 89.0% or more of the specified structure.

2. Density is 2.170 g / cm³ ３ The solid electrolyte according to claim 1.

3. It contains lithium, titanium, vanadium, and oxygen. Density is 2.170 g / cm³ ３ That concludes the discussion of solid electrolytes.

4. γ-Li ３ PO ４ The solid electrolyte according to claim 3, containing 89.0% or more of the type structure.

5. A solid electrolyte according to claim 1 or 3, represented by the following formula (1). (1) Li Ａ Today Ｂ V Ｃ O ４ (In equation (1) above, 3.40 ≤ A ≤ 3.70, 0.40 ≤ B ≤ 0.60, 0.40 ≤ C ≤ 0.60, and 0.70 ≤ B / C ≤ 1.40.)

6. The solid electrolyte according to claim 1 or 3, wherein the full width at half maximum of the main peak in the powder X-ray diffraction spectrum with CuKα as the source is 0.4° or less, and the diffraction angle 2θ is in the range of 21.0 to 21.7°.

7. The bulk ion conductivity of lithium ions at 25°C is 3.0 × 10⁻⁶. －５ The solid electrolyte according to claim 1 or 3, wherein the S / cm is 1 or higher.

8. The solid electrolyte according to claim 1 or 3, wherein no oxidation-reduction potential peak is observed in CV measurement in the range of 0 to 5 V.

9. A solid electrolyte sintered body comprising the solid electrolyte described in claim 1 or 3.

10. A lithium-ion secondary battery comprising the solid electrolyte described in claim 1 or 3.

11. The lithium-ion secondary battery according to claim 10, comprising the solid electrolyte as a solid electrolyte layer.

12. A step (1) of mixing a compound containing lithium, a compound containing titanium, and a compound containing vanadium, and A method for producing a solid electrolyte, comprising step (2) of calcining the mixture obtained in step (1) in a gas atmosphere with a carbon dioxide concentration of 300 ppm or less.

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