Solid electrolyte, solid electrolyte sintered body, and lithium-ion secondary battery

Abstract

To provide a novel solid electrolyte that can obtain high ionic conductivity when used as a material for lithium-ion secondary batteries, and a lithium-ion secondary battery containing the solid electrolyte. [Solution] A solid electrolyte comprising lithium, titanium, vanadium, oxygen, element M1, and element M2, wherein M1 is at least one element selected from silicon and germanium, and M2 is at least one element selected from group 15 elements, and is represented by the following formula (1). (1) Li A M1 B M2 C Ti D V E O4 [In the above formula (1), 3.40 ≤ A ≤ 3.70, 0

Description

[Technical Field]

[0001] This invention relates to a solid electrolyte, a solid electrolyte sintered body, and a lithium-ion secondary battery. [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] As a solid electrolyte, LISICON (Lithium Super Ionic Conductor) type materials are attracting attention and being investigated. 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. Summary of the Invention 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, for use as a solid electrolyte in a lithium ion battery that requires high battery performance, the ion conductivity is not sufficient and there is room for improvement.

[0008] An object of the present invention is to provide a novel solid electrolyte that can obtain high ion conductivity when used as a material for a lithium ion secondary battery, and a lithium ion secondary battery including the solid electrolyte. Means for Solving the Problems

[0009] A configuration example of the present invention for achieving the above object is as follows. [1] A solid electrolyte containing lithium, titanium, vanadium, oxygen, element M1, and element M2, where the element M1 is at least one element selected from silicon and germanium, the element M2 is at least one element selected from Group 15 elements, and represented by the following formula (1). (1)Li A M1 B M2 C Ti D V E O4 [In the formula (1), 3.40 ≤ A ≤ 3.70, 0 < B ≤ 0.450 - {0.250Si / (Si + Ge)} (in the above formula, Si / (Si + Ge) represents the content ratio (molar ratio) of silicon to the total of silicon and germanium in element M1), 0 < C ≤ 0.450, 0.50 ≤ D ≤ 0.500, 0.050 ≤ E ≤ 0.500, 0.70 ≤ B / C ≤ 1.40, and 0.70 ≤ D / E ≤ 1.40. ]

[0010] [2] The solid electrolyte according to [1], wherein the element M2 is phosphorus.

[0011] [3] The element M1 is silicon, In the formula (1), 0 < B ≤ 0.200, the solid electrolyte according to [1] or [2].

[0012] [4] The solid electrolyte according to any one of [1] to [3], containing a γ-Li3PO4 type structure.

[0013] [5] The solid electrolyte according to [4], containing 80% or more of the γ-Li3PO4 type structure.

[0014] [6] The density is 2.20 g / cm 3 or more, the solid electrolyte according to any one of [1] to [5].

[0015] [7] The bulk ion conductivity of lithium ions at 25 °C is 2.2 × 10 -5 S / cm or more, the solid electrolyte according to any one of [1] to [6].

[0016] [8] A solid electrolyte sintered body containing the solid electrolyte according to any one of [1] to [7].

[0017] [9] A lithium ion secondary battery containing the solid electrolyte according to any one of [1] to [7].

[0018]

[10] The lithium ion secondary battery according to [9], containing the solid electrolyte as a solid electrolyte layer.

Advantages of the Invention

[0019] The solid electrolyte in the preferred embodiment of the present invention has high ion conductivity. For example, when used as the solid electrolyte of a lithium ion secondary battery, a lithium ion secondary battery with high ion conductivity can be obtained.

Brief Description of the Drawings

[0020] [Figure 1] This is the XRD pattern of solid electrolyte 3 prepared in Example 3. [Figure 2] This is the XRD pattern of the solid electrolyte 5 prepared in Example 5. [Figure 3] This is the XRD pattern of the solid electrolyte 9 prepared in Example 9. [Figure 4] This is the XRD pattern of the solid electrolyte 16 prepared in Example 16. [Figure 5] This is the XRD pattern of the solid electrolyte 18 prepared in Example 18. [Figure 6] This is the XRD pattern of the solid electrolyte 22 prepared in Example 22. [Modes for carrying out the invention]

[0021] 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.

[0022] <Solid electrolyte> The solid electrolyte according to the present invention (hereinafter also referred to as "this solid electrolyte") is characterized by containing lithium, titanium, vanadium, oxygen, element M1, and element M2, and being represented by the following formula (1). Here, element M1 (hereinafter also simply referred to as "M1") is at least one element selected from silicon and germanium, and element M2 (hereinafter also simply referred to as "M2") is at least one element selected from Group 15 elements.

[0023] (1) Li A M1 B M2 C Ti D V E O4 [In the formula (1), 3.40 ≤ A ≤ 3.70, 0 < B ≤ 0.450 - {0.250Si / (Si + Ge)} (in the formula, Si / (Si + Ge) represents the content ratio (molar ratio) of silicon to the total of silicon and germanium in M1.), 0 < C ≤ 0.450, 0.050 ≤ D ≤ 0.500, 0.050 ≤ E ≤ 0.500, 0.70 ≤ B / C ≤ 1.40, and 0.70 ≤ D / E ≤ 1.40.]

[0024] A solid electrolyte is generally composed of a crystal structure formed from the contained elements. A solid electrolyte containing lithium, titanium, vanadium, and oxygen usually has a crystal structure containing lithium, titanium, vanadium, and oxygen (hereinafter also referred to as "LTVO structure") typified by the γ-Li3PO4 type structure described below. It is known that this solid electrolyte can increase the ionic conductivity of this solid electrolyte by doping M1 and M2 with different elements with respect to the LTVO structure.

[0025] A solid electrolyte having an LTVO structure is useful as a LISICON type material, and it is known that the LISICON type material has a feature of excellent thermal stability with a layered oxide cathode active material when used in a solid battery as compared with a NASICON type material (NAtrium Super Ionic CONductor) known as another solid electrolyte material ("Electrochemistry, 90(4), 336 - 341(2022)").

[0026] 〈Element M1〉 M1 is at least one element selected from silicon and germanium, and it may be only silicon, only germanium, or two types of silicon and germanium. This solid electrolyte can increase the ionic conductivity by containing M1. M1 is preferably only inexpensive silicon.

[0027] Although the mechanism by which the doping of the LTVO structure with M1 improves the ionic conductivity of the solid electrolyte is not clear, the inventors hypothesize that M1 can form a tetravalent cation with a large ionic radius, and that when M1 is substituted and dissolved in the LTVO structure, it results in an ionic radius different from that of the elements constituting the LTVO structure, causing lattice distortion in the crystal structure and reducing the resistance in the resistive layer within the crystal structure, thereby generating lithium ion conduction pathways and improving the ionic conductivity of the solid electrolyte.

[0028] <Element M2> M2 is at least one element selected from Group 15 elements, including nitrogen, phosphorus, arsenic, antimony, and bismuth. Preferably, it is the highly stable antimony or phosphorus, and more preferably, the low toxicity and highly stable phosphorus. Furthermore, phosphorus forms the tetrahedral crystal framework structure such as PO4, which facilitates the distribution of mobile ions in the ionic conduction pathways, resulting in high conductivity. This solid electrolyte can have high ionic conductivity by including M2. M2 may be present in one or more forms.

[0029] Although the mechanism by which the doping of M2 into the LTVO structure with a different element improves the ionic conductivity of the solid electrolyte is not clear, the inventors hypothesize that when M2 is substituted and dissolved into the LTVO structure, it results in an ionic radius different from that of the elements constituting the LTVO structure, causing lattice distortion in the crystal structure and reducing the resistance in the resistive layer within the crystal structure, thereby generating lithium ion conduction pathways and improving the ionic conductivity of the solid electrolyte.

[0030] This solid electrolyte is represented by the following formula (1). (1) Li A M1 B M2 C Ti D V E O4 [In the formula (1), 3.40 ≤ A ≤ 3.70, 0 < B ≤ 0.450 - {0.250Si / (Si + Ge)} (in the formula, Si / (Si + Ge) represents the content ratio (mol) of silicon to the total of silicon and germanium in M1), 0 < C ≤ 0.450, 0.050 ≤ D ≤ 0.500, 0.050 ≤ E ≤ 0.500, 0.70 ≤ B / C ≤ 1.40, and 0.70 ≤ D / E ≤ 1.40.]

[0031] In the formula (1), A represents the content (mol) of lithium in the solid electrolyte, which is 3.40 or more, preferably 3.45 or more, and more preferably 3.48 or more. Also, A is 3.70 or less, preferably 3.68 or less, and more preferably 3.66 or less. When the lithium content is within the above range, a solid electrolyte excellent in lithium ion conductivity can be obtained. The lower limit value and the upper limit value of A can be any combination.

[0032] In the formula (1), B represents the content (mol) of M1 in the solid electrolyte, which exceeds 0, preferably 0.001 or more, and more preferably 0.003 or more. The formula 0.450 - {0.250Si / (Si + Ge)} representing the upper limit value of B indicates that the upper limit value of B changes depending on the type and content of M1, preferably 0.350 - {0.230Si / (Si + Ge)}, and more preferably 0.300 - {0.220Si / (Si + Ge)}. When M1 is only silicon, that is, when Si / (Si + Ge) = 1, B is 0.200 or less, preferably 0.120 or less, and more preferably 0.080 or less. When M1 is only germanium, that is, when Si / (Si + Ge) = 0, B is 0.450 or less, preferably 0.350 or less, and more preferably 0.300 or less. When the content of M1 is within the above range, a lithium ion conduction path is formed by the skeletal lattice in which M1 is substituted and solid - dissolved in the LTVO structure, and a solid electrolyte excellent in lithium ion conductivity can be obtained. The lower limit value and the upper limit value of B can be any combination.

[0033] In formula (1) above, C represents the molar content of M2 in the solid electrolyte, and is greater than 0, preferably 0.001 or more, and more preferably 0.003 or more. Also, C is 0.450 or less, preferably 0.400 or less, more preferably 0.350 or less, and even more preferably 0.300 or less. When the M2 content is within the aforementioned range, lithium ion conduction pathways are formed by a skeletal lattice in which M2 is substituted and solid-solved in the LTVO structure, resulting in a solid electrolyte with excellent lithium ion conductivity. The lower limit and upper limit of C may be any combination.

[0034] In formula (1) above, the B / C ratio, which represents the content ratio of M1 to M2 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. When B / C is within the aforementioned range, a solid electrolyte with excellent lithium ion conductivity can be obtained. It is presumed that the amount of impurity phase consisting of M1 or M2 decreases, and lithium ion conductivity increases. Note that the lower limit and upper limit of B / C may be any combination.

[0035] In formula (1) above, D represents the titanium content (moles) in the solid electrolyte, and is 0.050 or more, preferably 0.100 or more, and more preferably 0.150 or more. Also, D is 0.500 or less, preferably 0.497 or less, and more preferably 0.495 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 D may be any combination.

[0036] In formula (1) above, E represents the vanadium content (moles) in the solid electrolyte, which is 0.050 or more, preferably 0.100 or more, and more preferably 0.150 or more. Also, D is 0.500 or less, preferably 0.497 or less, and more preferably 0.495 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 E may be any combination.

[0037] In formula (1) above, the D / E 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 D / E ratio is preferably 1.40 or lower, more preferably 1.30 or lower, and even more preferably 1.25 or lower. By having D / E 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 D / E may be any combination.

[0038] Since solid electrolytes generally have no overall charge, it is preferable that the relation A×1 + B×4 + C×5 + D×4 + E×5 - 2×4 = 0 is satisfied in equation (1) above. The above relation is valid because lithium is a monovalent cation, M1 is a tetravalent cation, M2 is a pentavalent cation, titanium is a tetravalent cation, vanadium is a pentavalent cation, and oxygen is a divalent anion.

[0039] Furthermore, substitutional solid solutions resulting from the doping of M1 and M2 with different elements in the LTVO structure generally occur between M1, M2, titanium, and vanadium. For this reason, in equation (1), it is preferable that B to E satisfy the relationship B + C + D + E = 1. This relationship holds because the total content of transition metals relative to lithium and oxygen theoretically does not change.

[0040] In equation (1) above, the values ​​A to E, that is, the ratio of the number of atoms of each constituent element of lithium, titanium, vanadium, M1, M2, and oxygen that constitute 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.

[0041] <Crystal structure> The crystalline structure constituting this solid electrolyte is formed by doping M1 and M2 with different elements to the LTVO structure. Preferably, the crystalline structure constituting this solid electrolyte contains a γ-Li3PO4 type structure. The proportion of the γ-Li3PO4 type structure in this solid electrolyte is preferably 80% or more, more preferably 85% or more, and even more preferably 87% or more. The inclusion of the γ-Li3PO4 type structure is advantageous for lithium ion conduction, and because the γ-Li3PO4 type structure does not react easily with the active material, it has high electrical stability, which is advantageous for battery application. The upper limit of the proportion of the γ-Li3PO4 type structure in this solid electrolyte is 100%. The percentage of the γ-Li3PO4 type structure in this solid electrolyte was determined by powder X-ray diffraction spectroscopy using Cu-Kα radiation as the source, specifically by the method described in the examples.

[0042] <density> The density of this solid electrolyte is preferably 2.20 g / cm³. 3 More preferably 2.21 g / cm³ 3 More preferably 2.22 g / cm³ 3 This concludes the explanation. 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 mechanical strength also increases, contributing to improved battery durability. The upper limit of the density of this solid electrolyte is not particularly limited, but for example, 2.50 g / cm³ is used. 3 That is the case. The density of this solid electrolyte can be determined by the method described in the examples.

[0043] <Relative density> The relative density of this solid electrolyte is preferably 80% or higher, more preferably 85% or higher, and even more preferably 87% 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.

[0044] <Bulk ion conductivity> The bulk ion conductivity of this solid electrolyte is preferably 2.2 × 10⁻⁶. -5 S / cm or more, more preferably 2.4 × 10 -5 S / cm or more, more preferably 2.5 × 10 -5 S / cm or more, particularly preferably 3.0 × 10 -5 The 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.

[0045] 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.

[0046] <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.

[0047] In the manufacturing method of this solid electrolyte, methods such as solid-phase reactions and liquid-phase reactions can be employed. The manufacturing method by solid-phase reaction will be described in detail below. Examples of manufacturing methods using solid-phase reactions include those having at least one mixing step and one calcination step.

[0048] (Mixing process) In the mixing step, a compound containing lithium atoms, a compound containing titanium atoms, a compound containing vanadium atoms, a compound containing M1, and a compound containing M2 are mixed. Note that lithium atoms, titanium atoms, vanadium atoms, M1, and M2 may be present in two or more atoms simultaneously in a single compound. If the mixing process is performed multiple times, the mixing process may include a stage in which only a portion of the above compound is present. Furthermore, if the mixing process is performed multiple times, the second and subsequent mixing processes may be carried out after the calcination process described later, or after any crushing or pulverizing process. In other words, the remaining compound may be mixed with the calcination product after the calcination process described later, or with the powder after any crushing or pulverizing process, using the mixture after the mixing process in which only a portion of the above compound is present.

[0049] 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.

[0050] While there are no particular limitations on the titanium atom-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.

[0051] While there are no particular limitations on the compounds containing vanadium atoms, inorganic compounds are preferred for ease of handling, and examples of vanadium compounds include vanadium(V) oxide and ammonium vanadate. For ease of handling, the use of vanadium(V) oxide is more preferable. These vanadium compounds may be used individually or in combination of two or more.

[0052] The compound containing M1 is not particularly limited as long as it contains one or more elements selected from the group consisting of silicon and germanium. However, inorganic compounds are preferred in terms of ease of handling, inorganic oxides containing one or more elements selected from the group consisting of silicon and germanium are more preferred, and inorganic oxides are even more preferred. Specific examples of the inorganic oxide include silicon oxide (SiO2) and germanium oxide (GeO2). These substances may be used individually or in combination of two or more.

[0053] The compound containing M2 is not particularly limited as long as it contains one or more elements selected from Group 15 elements, but inorganic compounds are preferred in terms of ease of handling, inorganic oxides or inorganic hydroxides containing one or more elements selected from Group 15 elements or elements in their elemental form are more preferred, and inorganic oxides are even more preferred. Specifically, lithium phosphate (Li3PO4) can be given as the inorganic oxide. These substances may be used individually or in combination of two or more.

[0054] 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.

[0055] When B is adjusted to fall within the range detailed in equation (1) above, it is thought that when it is substituted and solid-solved in the LTVO structure, it forms a skeletal lattice, enabling the formation of lithium ion conduction pathways, while suppressing the formation of impurity phases consisting of M1 and lithium, thus improving lithium ion conductivity. When C is adjusted to fall within the range detailed in equation (1) above, it is thought that when it is substituted and dissolved in the LTVO structure, it forms a skeletal lattice, enabling the formation of lithium ion conduction pathways, while suppressing the formation of impurity phases consisting of M2 and lithium, thus improving lithium ion conductivity. From the description in Non-Patent Document 1, it can be inferred that when D / E 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.

[0056] 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. A solvent may be added during the mixing process. Adding a solvent improves the uniformity of the mixture. Examples of solvents include pure water.

[0057] The mixing process may be carried out under an atmospheric environment. A gas atmosphere of nitrogen gas and / or argon gas is more preferable.

[0058] (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.

[0059] (Firing process) In the firing process, the mixture obtained in the mixing process is fired. 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 ensures sufficient solid solution of M1 and M2, improving ionic conductivity, while firing at 1200°C or lower is preferable as it prevents lithium atoms from easily leaching out of the system.

[0060] 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 be high 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.

[0061] 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).

[0062] If the firing process is carried out in multiple stages, a crushing / grinding 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. The firing process may be carried out in the atmosphere, but it is preferable to be carried out in a gas atmosphere with a controlled carbon dioxide concentration. Examples of gases used include those with a carbon dioxide concentration adjusted to, for example, 300 ppm or less, preferably 100 ppm, more preferably 50 ppm or less, even more preferably 10 ppm or less, and particularly preferably 1 ppm or less. The gas used can be any known form of gas with a controlled carbon dioxide concentration, such as pure air (a mixed gas of O2:N2=20:80) or dry air (compressed air), and may also be an inert gas such as nitrogen gas or argon gas. Furthermore, this solid electrolyte exhibits superior stability during firing compared to conventionally known solid electrolytes having an LTVO structure. Therefore, even when firing is performed under atmospheric conditions, a solid electrolyte with sufficient ion conductivity can be easily obtained.

[0063] 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 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 firing time is preferably 1 to 8 hours, and preferably 2 to 6 hours. Note that "high-temperature firing twice" includes a step of lowering the temperature to room temperature between the first and second firings.

[0064] 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.

[0065] (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.

[0066] <Solid Electrolyte Sintered Body> The solid electrolyte sintered body according to the present invention (hereinafter also referred to as "the solid electrolyte sintered body") contains the solid electrolyte, and preferably consists of the solid electrolyte.

[0067] <Method for manufacturing solid electrolyte sintered bodies> The method for manufacturing a solid electrolyte sintered body 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. A manufacturing method for this solid electrolyte sintered body includes a manufacturing method having at least one molding step and at least one sintering step.

[0068] (molding process) In the molding process, the solid electrolyte obtained by the method for manufacturing the solid electrolyte described above is molded. 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.

[0069] 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.

[0070] The molding process may be carried out under an atmospheric environment. A gas atmosphere of nitrogen gas and / or argon gas is more preferable.

[0071] (Sintering process) In the sintering process, the molded body obtained in the molding process is sintered. 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 is preferable because it allows for sufficient solid solution of M1 and M2, improving ionic conductivity, while sintering at 1200°C or lower is preferable because it prevents lithium atoms from easily leaching out of the system.

[0072] 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 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.

[0073] The sintering process may be carried out in multiple stages. For example, if a two-stage sintering process is performed, 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 firing temperature (e.g., 700-1200°C) and firing time (e.g., 1-8 hours). If the sintering process is carried out in multiple stages, a crushing / grinding step 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. The sintering time and sintering temperature are adjusted in coordination with each other.

[0074] The sintering process may be carried out in an atmospheric environment, but it is preferable to carry it out in a gas atmosphere with a controlled carbon dioxide concentration. The carbon dioxide concentration of the gas used is the same as that described in the firing process. The gas used can be any known form of gas with a controlled carbon dioxide concentration, such as pure air (a mixed gas of O2:N2=20:80) or dry air, and may also be an inert gas such as nitrogen gas or argon gas.

[0075] 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. Note that "high-temperature sintering twice" includes a step of lowering the temperature to room temperature between the first and second sintering steps.

[0076] 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 during the cooling process after sintering. In this way, the solid electrolyte sintered body can be obtained.

[0077] <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 such that a positive electrode current collector, a positive electrode layer, the solid electrolyte layer, a negative electrode layer, and a negative electrode current collector are stacked in this order.

[0078] The positive current collector and the negative current collector are not particularly limited as long as the material thereof conducts electrons without causing an electrochemical reaction. Examples include simple metals and alloys such as copper, aluminum, and iron, or conductors such as conductive metal oxides such as antimony-doped tin oxide (ATO) and tin-doped indium oxide (ITO). In addition, a current collector provided with a conductive adhesive layer on the surface of these conductors can also be used. The conductive adhesive layer can be configured to include a granular conductive material, a fibrous conductive material, or the like.

[0079] As the active material for the negative electrode layer, a lithium alloy, a metal oxide, graphite, hard carbon, soft carbon, silicon, a silicon alloy, silicon oxide SiO n (0 < n ≤ 2), 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, and the like can be mentioned.

[0080] A composite material in which a silicon / carbon composite material or 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 accompanying the lithium storage / discharge of silicon, and can maintain a good balance of macro-conductivity, micro-conductivity, and ion conductivity in the composite electrode material or the 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.

[0081] Examples of active materials for positive electrodes include those containing at least one selected from the group consisting of LiCo oxide, LiNiCo oxide, LiNiCoMn oxide, LiNiMn oxide, LiMn oxide, LiMn-based spinel, LiMnNi oxide, LiMnAl oxide, LiMnMg oxide, LiMnCo oxide, LiMnFe oxide, LiMnZn oxide, LiCrNiMn oxide, LiCrMn oxide, lithium titanate, lithium metallic 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-storable intermetallic compounds.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] (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.

[0086] 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.

[0087] 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.

[0088] 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]

[0089] The following describes examples of the present invention in detail with reference to embodiments, but the present invention is not limited thereto.

[0090] [Example 1] In a 2000mL polypropylene bottle, the following ingredients are used: 400g of pure water, 45.000g (1.07245mol) of lithium hydroxide monohydrate (grade: Kagoshima special grade, purity: >98.0%, manufactured by Kanto Chemical Co., Ltd.), 13.191g (0.07252mol) of vanadium(V) oxide (grade: Kagoshima special grade, purity: >99.0%, manufactured by Kanto Chemical Co., Ltd.), and 1g of titanium dioxide (grade: F-6A, purity: >99%, manufactured by Resonaq Ceramics Co., Ltd.). 1.585g (0.14505mol), 0.088g (0.00146mol) of fumed silica (SiO2) (AEROSIL200, purity: >99.9%, manufactured by Nippon Aerosil Co., Ltd.), 0.170g (0.00147mol) of lithium phosphate (grade: 1st grade, purity: >95.0%, manufactured by Kanto Chemical Co., Ltd.), and 2800g of 5mm diameter zirconia balls were added and mixed using a pot mill at 300rpm for 24 hours. 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 stainless steel pot and dried using a hot stirrer at 130°C, 100 rpm, for 20 hours. After drying, the collected powder was ground in a mortar. 10g of the crushed mixed powder was placed on an alumina setter and heated to 900°C at a heating rate of 5°C / min in a quartz tube furnace with an outer diameter of 160mm (inner diameter of 154mm) under a pure air atmosphere (carbon dioxide concentration <1ppm), held for 3 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 3000 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 (1) (B=C, D=E, (B+C):(D+E)=1:99), Li 3.500 Si 0.005 P 0.005 Ti 0.495 V 0.495 O 4.000 It has the composition represented by [the formula]. (1) Li A M1 B M2 C Ti D V E O4 (In equation (1) above, A = -5(C+E)-4(B+D)+8 and B+C+D+E=1. In equation (1) above, A represents the lithium content in the solid electrolyte and is determined from the previously determined B, C, D, E and the valencies of the ions so that the total charge is 0.)

[0091] [Example 2] A powder of solid electrolyte 2 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 13.499 g (0.07422 mol) of vanadium(V) oxide, 11.856 g (0.14844 mol) of titanium dioxide, 0.182 g (0.00303 mol) of fumed silica, and 0.351 g (0.00303 mol) of lithium phosphate were used as raw materials. Solid electrolyte 2 is calculated from the above formula (1) (B=C, D=E, (B+C):(D+E)=2:98), Li 3.500 Si 0.010 P 0.010 Ti 0.490 V 0.490 O 4.000 It has the composition represented by [the formula].

[0092] [Example 3] A powder of solid electrolyte 3 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 13.418 g (0.07378 mol) of vanadium(V) oxide, 11.785 g (0.14755 mol) of titanium dioxide, 0.274 g (0.00456 mol) of fumed silica, and 0.528 g (0.00456 mol) of lithium phosphate were used as raw materials. The solid electrolyte 3 is calculated from the above formula (1) (B=C, D=E, (B+C):(D+E)=3:97), Li 3.500 Si 0.015 P 0.015 Ti 0.485 V 0.485 O 4.000 It has the composition represented by [the formula].

[0093] [Example 4] A powder of solid electrolyte 5 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 13.338 g (0.07333 mol) of vanadium(V) oxide, 11.714 g (0.14667 mol) of titanium dioxide, 0.367 g (0.00611 mol) of fumed silica, and 0.708 g (0.00611 mol) of lithium phosphate were used as raw materials. The solid electrolyte 4 is calculated from the above formula (1) (B=C, D=E, (B+C):(D+E)=4:96), Li 3.500 Si 0.020 P 0.020 Ti 0.480 V 0.480 O 4.000 It has the composition represented by [the formula].

[0094] [Example 5] A powder of solid electrolyte 5 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 12.869 g (0.07075 mol) of vanadium(V) oxide, 11.302 g (0.14151 mol) of titanium dioxide, 0.448 g (0.00745 mol) of fumed silica, and 0.862 g (0.00745 mol) of lithium phosphate were used as raw materials. The solid electrolyte 5 is calculated from the above formula (1) (B=C, D=E, (B+C):(D+E)=5:95), Li 3.500 Si 0.025 P 0.025 Ti 0.475 V 0.475 O 4.000 It has the composition represented by [the formula].

[0095] [Example 6] A powder of solid electrolyte 6 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 12.451 g (0.06846 mol) of vanadium(V) oxide, 10.935 g (0.13692 mol) of titanium dioxide, 0.914 g (0.01521 mol) of fumed silica, and 1.761 g (0.01521 mol) of lithium phosphate were used as raw materials. The solid electrolyte 6 is calculated from the above formula (1) (B=C, D=E, (B+C):(D+E)=10:90), Li 3.500 Si 0.050 P 0.050 Ti 0.450 V 0.450 O 4.000 It has the composition represented by [the formula].

[0096] [Example 7] A powder of solid electrolyte 7 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 12.015 g (0.06606 mol) of vanadium(V) oxide, 10.552 g (0.13212 mol) of titanium dioxide, 1.401 g (0.02331 mol) of fumed silica, and 2.700 g (0.02332 mol) of lithium phosphate were used as raw materials. The solid electrolyte 7 is calculated from the above formula (1) (B=C, D=E, (B+C):(D+E)=15:85), Li 3.500 Si 0.075 P 0.075 Ti 0.425 V 0.425 O 4.000 It has the composition represented by [the formula].

[0097] [Example 8] (Preparation of Solid Electrolyte) Powder of solid electrolyte 8 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 11.558 g (0.06355 mol) of vanadium(V) oxide, 10.151 g (0.12710 mol) of titanium oxide, 1.910 g (0.03177 mol) of fumed silica, and 3.679 g (0.03177 mol) of lithium phosphate were used as raw materials. Solid electrolyte 8, calculated from the above formula (1) (B = C, D = E, (B + C):(D + E) = 20:80), is Li 3.500 Si 0.100 P 0.100 Ti 0.400 V 0.400 O 4.000 and has a composition represented by.

[0098] [Example 9] Powder of solid electrolyte 9 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 12.869 g (0.07075 mol) of vanadium(V) oxide, 11.302 g (0.14151 mol) of titanium oxide, 0.862 g (0.00745 mol) of lithium phosphate, and 0.779 g (0.00745 mol) of germanium(IV) oxide (grade: high purity reagent, purity: >99.99%, manufactured by Kanto Chemical Co., Inc.) were used instead of fumed silica as raw materials. Solid electrolyte 9, calculated from the above formula (1) (B = C, D = E, (B + C):(D + E) = 5:95), is Li 3.500 Ge 0.025 P 0.025 Ti 0.475 V 0.475 O 4.000 and has a composition represented by.

[0099] [Example 10] A powder of solid electrolyte 10 was obtained in the same manner as in Example 9, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 12.451 g (0.06846 mol) of vanadium(V) oxide, 10.935 g (0.13692 mol) of titanium oxide, 1.591 g (0.01521 mol) of germanium(IV) oxide, and 1.761 g (0.01521 mol) of lithium phosphate were used as raw materials. The solid electrolyte 10 has a composition represented by Li calculated from the above formula (1) (B = C, D = E, (B + C):(D + E) = 10:90), 3.500 Ge 0.050 P 0.050 Ti 0.450 V 0.450 O 4.000 as follows.

[0100] [Example 11] A powder of solid electrolyte 11 was obtained in the same manner as in Example 9, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 11.558 g (0.06355 mol) of vanadium(V) oxide, 10.151 g (0.12710 mol) of titanium oxide, 3.324 g (0.03178 mol) of germanium(IV) oxide, and 3.679 g (0.03177 mol) of lithium phosphate were used as raw materials. The solid electrolyte 11 has a composition represented by Li calculated from the above formula (1) (B = C, D = E, (B + C):(D + E) = 20:80), 3.500 Ge 0.100 P 0.100 Ti 0.400 V 0.400 O 4.000 as follows.

[0101] [Example 12] A powder of solid electrolyte 12 was obtained in the same manner as in Example 9, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 8.336 g (0.04583 mol) of vanadium(V) oxide, 7.321 g (0.09166 mol) of titanium oxide, 9.589 g (0.09166 mol) of germanium(IV) oxide, and 10.614 g (0.09167 mol) of lithium phosphate were used as raw materials. The solid electrolyte 12 is calculated from the above formula (1) (B=C, D=E, (B+C):(D+E)=50:50), Li 3.500 Ge 0.250 P 0.250 Ti 0.250 V 0.250 O 4.000 It has the composition represented by [the formula].

[0102] [Example 13] A powder of solid electrolyte 13 was obtained in the same manner as in Example 9, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 3.941 g (0.02167 mol) of vanadium(V) oxide, 3.461 g (0.04333 mol) of titanium dioxide, 18.132 g (0.17333 mol) of germanium(IV) oxide, and 20.069 g (0.17333 mol) of lithium phosphate were used as raw materials. The solid electrolyte 13 is calculated from the above formula (1) (B=C, D=E, (B+C):(D+E)=80:20), Li 3.500 Ge 0.400 P 0.400 Ti 0.100 V 0.100 O 4.000 It has the composition represented by [the formula].

[0103] [Comparative Example 1] A powder of solid electrolyte 27 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 13.933 g (0.07660 mol) of vanadium(V) oxide, and 12.237 g (0.15321 mol) of titanium dioxide were used as raw materials. The solid electrolyte 27 is calculated from the above formula (1) (B=C=0, D=E=0.500), Li 3.500 Ti 0.500 V 0.500 O 4.000 It has the composition represented by [the formula].

[0104] [Comparative Example 2] A powder of solid electrolyte 28 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 8.336 g (0.04583 mol) of vanadium(V) oxide, 7.321 g (0.09166 mol) of titanium dioxide, 5.509 g (0.09166 mol) of fumed silica, and 10.614 g (0.09167 mol) of lithium phosphate were used as raw materials. The solid electrolyte 28 is calculated from the above formula (1) (B=C, D=E, (B+C):(D+E)=50:50), Li 3.500 Si 0.250 P 0.250 Ti 0.250 V 0.250 O 4.000 It has the composition represented by [the formula].

[0105] [Comparative Example 3] A powder of solid electrolyte 29 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 3.941 g (0.02167 mol) of vanadium(V) oxide, 3.461 g (0.04333 mol) of titanium dioxide, 10.417 g (0.17333 mol) of fumed silica, and 20.069 g (0.17333 mol) of lithium phosphate were used as raw materials. The solid electrolyte 29 is calculated from the above formula (1) (B=C, D=E, (B+C):(D+E)=80:20), Li 3.500 Si 0.400 P 0.400 Ti 0.100 V 0.100 O 4.000 It has the composition represented by [the formula].

[0106] [Comparative Example 4] A powder of solid electrolyte 30 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 14.816 g (0.24653 mol) of fumed silica, and 28.546 g (0.24653 mol) of lithium phosphate were used as raw materials. The solid electrolyte 30 is calculated from the above formula (1) (B=C=0.500, D=E=0), Li 3.500 Si 0.500 P0.500 O 4.000 It has the composition represented by [the formula].

[0107] [Comparative Example 5] A powder of solid electrolyte 31 was obtained in the same manner as in Example 1, except that 45.000 g (1.07245 mol) of lithium hydroxide monohydrate, 25.789 g (0.24653 mol) of germanium(IV) oxide, and 28.546 g (0.24653 mol) of lithium phosphate were used as raw materials. The solid electrolyte 31 is calculated from the above formula (1) (B=C=0.500, D=E=0), Li 3.500 Ge 0.500 P 0.500 O 4.000 It has the composition represented by [the formula].

[0108] [Example 14] A powder of solid electrolyte 14 was obtained in the same manner as in Example 1, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0109] [Example 15] A powder of solid electrolyte 15 was obtained in the same manner as in Example 2, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0110] [Example 16] A powder of solid electrolyte 16 was obtained in the same manner as in Example 3, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0111] [Example 17] A powder of solid electrolyte 17 was obtained in the same manner as in Example 4, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0112] [Example 18] A powder of solid electrolyte 18 was obtained in the same manner as in Example 5, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0113] [Example 19] A powder of solid electrolyte 19 was obtained in the same manner as in Example 6, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0114] [Example 20] A powder of solid electrolyte 20 was obtained in the same manner as in Example 7, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0115] [Example 21] A powder of solid electrolyte 21 was obtained in the same manner as in Example 8, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0116] [Example 22] A powder of solid electrolyte 22 was obtained in the same manner as in Example 9, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0117] [Example 23] A powder of solid electrolyte 23 was obtained in the same manner as in Example 10, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0118] [Example 24] A powder of solid electrolyte 24 was obtained in the same manner as in Example 11, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0119] [Example 25] A powder of solid electrolyte 25 was obtained in the same manner as in Example 12, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0120] [Example 26] A powder of solid electrolyte 26 was obtained in the same manner as in Example 13, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0121] [Comparative Example 6] A powder of solid electrolyte 32 was obtained in the same manner as in Comparative Example 1, except that 10 g of the pulverized mixed powder was heated to 900°C in an air atmosphere at a heating rate of 5°C / min, held for 2 hours, and then allowed to cool naturally.

[0122] [Measurement and evaluation of solid electrolytes] The solid electrolytes produced in the above examples and comparative examples were measured and evaluated for the following items. The results of these evaluations are summarized in Table 1 below.

[0123] (Fabrication of solid electrolyte sintered bodies) 8 g of the solid electrolyte powder prepared in the example or comparative example and 380 g of φ3 mm zirconia balls were placed in a 250 mL polypropylene pot and ground for 1 hour using a vibratory mill (paint shaker, manufactured by Asada Iron Works Co., Ltd.). 0.95 g of the solid electrolyte powder recovered after grinding was mixed with 0.50 g of a 10 wt% acrylic molding aid ethanol solution and mixed using a mortar. 0.3 g of the solid electrolyte powder with the molding aid added 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 400°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 a pure air atmosphere (carbon dioxide concentration < 1 ppm), held for 1 hour, then heated to 800°C and held for 3 hours, and then allowed to cool naturally to obtain a pellet-shaped solid electrolyte sintered body. The solid electrolyte 14 was calcined under atmospheric conditions at 725°C for a holding time of 2 hours.

[0124] <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 measured using an X'Pert PRO X-ray diffraction analyzer (PANalytical), using Cu-Kα rays (output: 45kV, 40mA), diffraction angle 2θ: range of 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 electrolytes 3, 5, 9, 16, 18, and 22 are shown in Figures 1 to 6, respectively. From the obtained XRD figures, the height of the main peak (21°≦2θ≦22°) of the γ-Li3PO4 type structure and the sum of the heights of the main peaks (Li2CO3: 30°≦2θ≦32°, Li2O: 43°≦2θ≦44°, LiTiO2: 43°≦2θ≦44°) of each crystal structure considered to be present in addition to the γ-Li3PO4 type structure were used to calculate the percentage of the γ-Li3PO4 type structure in the solid electrolyte according to the following formula.

[0125]

number

[0126] <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 at four points 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.

[0127]

number

[0128] 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.

[0129]

number

[0130]

number

[0131] <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 pellet sintered body were covered with masking tape, and gold electrodes were formed on both sides of the pellet using a vapor deposition apparatus (QUICK AUTO COATER SC-701AT). After electrode formation, the masking tape on the sides of the pellet sintered body was removed, and the pellet 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 the frequency range of 1 MHz to 1 Hz and with an applied voltage of 100 mV. The bulk ionic conductivity (S / cm) of the solid electrolyte was calculated using the following formula.

[0132]

number

[0133] [Table 1]

[0134] The solid electrolyte represented by formula (1) above was shown to have higher bulk ion conductivity compared to the solid electrolytes of Comparative Examples 1 to 5.

[0135] The solid electrolyte represented by formula (1) above, even when fired in an air atmosphere (Examples 14-26), contains Li 3.500 Ti 0.500 V 0.500 O 4.000Compared to Comparative Example 6, in which the solid electrolyte represented by formula (1) was calcined under an atmospheric environment, the proportion (%) of the γ-Li3PO4 type structure was higher. This demonstrates that the solid electrolyte represented by formula (1) exhibits excellent stability during calcination, and that a solid electrolyte with sufficient ionic conductivity can be easily obtained even when calcined under an atmospheric environment.

Claims

1. It contains lithium, titanium, vanadium, oxygen, element M1, and element M2. The element M1 is at least one element selected from silicon and germanium. The element M2 is at least one element selected from Group 15 elements, A solid electrolyte represented by the following formula (1). (1) i Ａ 71 Ｂ 72 Ｃ Ti Ｄ 6 Ｅ 9 ４ [In formula (1) above, 3.40 ≤ A ≤ 3.70, 0 < B ≤ 0.450 - {0.250Si / (Si + Ge)} (wherein Si / (Si + Ge) represents the molar ratio of silicon to the total amount of silicon and germanium in element M1), 0 < C ≤ 0.450, 0.50 ≤ D ≤ 0.500, 0.050 ≤ E ≤ 0.500, 0.70 ≤ B / C ≤ 1.40, and 0.70 ≤ D / E ≤ 1.40.]

2. The solid electrolyte according to claim 1, wherein the element M2 is phosphorus.

3. The element M1 is silicon, The solid electrolyte according to claim 1 or 2, wherein in formula (1), 0 < B ≤ 0.

200.

4. γ-Li ３ PO ４ A solid electrolyte according to claim 1 or 2, comprising a type structure.

5. The above-mentioned γ-Li ３ PO ４ type structure, and the solid electrolyte according to claim 4 containing 80% or more thereof.

6. Density is 2.20 g / cm³ ３ The solid electrolyte according to claim 1 or 2.

7. The bulk ion conductivity of lithium ions at 25°C is 2.2 × 10⁻⁶. －５ The solid electrolyte according to claim 1 or 2, wherein the S / cm is 1 or higher.

8. A solid electrolyte sintered body comprising the solid electrolyte described in claim 1 or 2.

9. A lithium-ion secondary battery comprising the solid electrolyte described in claim 1 or 2.

10. The lithium-ion secondary battery according to claim 9, comprising the solid electrolyte as a solid electrolyte layer.

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