Lithium ion conductive solid electrolyte

Abstract

The present invention provides an oxide-based lithium-ion conductive solid electrolyte having a high ionic conductivity, and a lithium-ion secondary battery using the solid electrolyte. The present invention includes a lithium-ion conductive solid electrolyte composed of a compound represented by the compositional formula Li 2‑x Ti 1‑x M1 x O3, where M1 is at least one metal element selected from the group consisting of niobium and tantalum elements, and 0.05 ≤ x ≤ 0.15.

Description

Technical Field

[0001] The present invention relates to a lithium ion conductive solid electrolyte. Background Art

[0002] In recent years, there has been a demand for the development of high-output and high-capacity batteries as power sources for laptop computers, tablet terminals, mobile phones, smartphones, and electric vehicles (EVs), etc. Among them, all-solid-state lithium-ion batteries that use solid electrolytes instead of liquid electrolytes such as organic solvents have attracted attention due to their excellent charge and discharge efficiency, charging speed, safety, and productivity.

[0003] In the field of such all-solid lithium ion batteries, technologies for improving positive electrode materials and the like using lithium ion conductor materials have been disclosed.

[0004] For example, Patent Document 1 discloses a positive electrode which reduces the interface resistance by disposing a coating film containing a lithium ion conductor on the surface of the positive electrode, thereby suppressing the diffusion of components such as Co, P, and S at the interface between the positive electrode active material and the sulfide-based solid electrolyte, preventing the formation of a lithium-deficient layer. In addition, Patent Document 2 discloses a positive electrode active material which reduces the interface resistance by disposing a coating film containing LiNbO on the surface of the positive electrode active material. 3 The reaction suppression unit of the first lithium ion conductor having good Li ion conductivity and the second lithium ion conductor having high electrochemical stability can suppress the increase in interface resistance between the positive electrode active material and the sulfide solid electrolyte material over time.

[0005] Prior art literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2021-150286

[0007] Patent Document 2: Japanese Patent Application Publication No. 2013-026003 Summary of the invention

[0008] As a lithium ion conductive solid electrolyte that conducts lithium ions, an oxide-based solid electrolyte is more preferable than a sulfide-based solid electrolyte from the viewpoint of safety.

[0009] However, although Patent Document 1 exemplifies lithium zirconium oxide, lithium titanium oxide, lithium niobium oxide, etc. as the lithium ion conductor, it does not disclose Li 2 O-ZrO 2 In addition, Patent Document 2 exemplifies LiNbO as the first lithium ion conductor. 3 Niobium oxide, LiTaO 3 A Li-containing oxide such as tantalum oxide, etc., and a Li-containing oxide such as tantalum oxide, etc. are exemplified as the second lithium ion conductor. 2 Ti 2 O5 , Li 2 Ti 2 O 3 , Li 4 Ti 5 O 12 A Li-containing compound having a polyanionic structural part containing at least one of B, Si, P, Al and W, etc.

[0010] However, in these documents, there is neither description nor suggestion of improving the ion conductivity of oxide-based solid electrolytes as lithium ion conductors. In view of the above situation, an object of the present invention is to provide an oxide-based lithium ion conductive solid electrolyte with high ion conductivity, and a lithium ion secondary battery using the solid electrolyte.

[0011] The present invention includes the following aspects. [1]

[0013] A lithium ion conductive solid electrolyte,

[0014] It is composed of the formula Li 2-x Ti 1-x M1 x O 3 The compound represented by

[0015] The M1 is at least one metal element selected from the group consisting of niobium and tantalum,

[0016] 0.05≤x≤0.15. [2]

[0018] According to the lithium ion conductive solid electrolyte described in [1] above, a monoclinic crystal structure can be confirmed by X-ray diffraction measurement. [3]

[0020] According to the lithium ion conductive solid electrolyte described in [1] or [2], the total lithium ion conductivity σ total (25℃) is 1.0×10 -6 (S / cm) or more. [4]

[0022] A lithium ion secondary battery comprising the lithium ion conductive solid electrolyte described in any one of [1] to [3] as a solid electrolyte.

[0023] According to the present invention, there can be provided an oxide-based lithium ion conductive solid electrolyte having high ion conductivity, and a lithium ion secondary battery using the solid electrolyte.

[0024] Furthermore, by using the lithium ion conductive solid electrolyte, for example, a high-output and high-capacity lithium ion secondary battery can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 These are X-ray diffraction patterns of lithium ion conductive solid electrolytes (1) to (3) prepared in Example 1 and Comparative Examples 1 and 2, respectively. DETAILED DESCRIPTION

[0026] A lithium ion conductive solid electrolyte (hereinafter also referred to as the present electrolyte) according to one embodiment of the present invention is a solid electrolyte having a composition formula Li 2-x Ti 1-x M1 x O 3 The compound represented by: M1 is at least one metal element selected from niobium and tantalum, and 0.05≤x≤0.15.

[0027] More specifically, the present electrolyte is composed of the formula Li 2 TiO 3 The electrolyte is partially replaced by the M1 element doped with the titanium element in the lithium-containing titanium oxide represented by . Here, it is considered that when the M1 element replaces the tetravalent titanium ions with pentavalent ions, as shown in the lithium composition represented by 2-x, from the viewpoint of charge compensation, lithium vacancies where the lithium element does not exist in the compound are introduced into the crystal structure, and lithium ions become easy to move, thereby improving the lithium ion conductivity.

[0028] (Constituent elements of lithium ion conductive solid electrolyte)

[0029] The present electrolyte has at least lithium, titanium, M1 and oxygen as constituent elements, and M1 is at least one metal element selected from niobium and tantalum. The present electrolyte can also be said to be a lithium ion conductive solid electrolyte composed of a specific oxide containing lithium. However, this does not strictly exclude the presence of impurities in the lithium ion conductive solid electrolyte. Unavoidable impurities arising from raw materials and / or manufacturing processes, impurities with other crystal systems within a range that does not deteriorate lithium ion conductivity, etc. may also be included in the lithium ion conductive solid electrolyte.

[0030] The ratio of the number of atoms of each constituent element of lithium, titanium, Mn and oxygen constituting the lithium ion conductive solid electrolyte in the preferred embodiment of the present invention can be measured by using a standard powder sample containing Mn, Co and Ni in a ratio of 1:1:1 by using an absolute intensity quantitative method using Auger electron spectroscopy (AES). As a standard powder sample, for example, LiCoO 2 Etc. Lithium-containing transition metal oxides, etc.

[0031] (M1 content)

[0032] The composition formula of the present electrolyte is represented by the following formula (1).

[0033] Li 2-x Ti 1-x M1 x O 3 Formula (1)

[0034] In the above formula (1), the content of M1 contained in the present electrolyte represented by x is greater than 0.05 and less than 0.15. If it is expressed as a percentage of the number of atoms of M1 relative to the total number of atoms of titanium and M1, the range of the content is greater than 5% and less than 15%. When represented by x in the above formula (1), the lower limit of the M1 content is preferably 0.06, more preferably 0.07, and particularly preferably 0.08. When represented by x in the above formula (1), the upper limit of the M1 content is preferably 0.13, more preferably 0.12, and further preferably 0.10. If the M1 content is within the above range, the lithium ion conductivity is greatly improved. The M1 content can be obtained by quantitative analysis known in the past as a percentage of the number of atoms of M1 relative to the total number of atoms of titanium and M1. For example, the M1 content can be obtained by adding an acid to the sample and thermally decomposing it, then fixing the volume of the thermal decomposition product and using a high-frequency inductively coupled plasma (ICP) emission analysis device. In the method for producing a lithium ion conductive solid electrolyte described below, since titanium and M1 do not flow out of the system, the percentage of the number of atoms of M1 relative to the total number of atoms of titanium and M1, which represents the doping amount of M1, can be simply calculated from the charged amounts of raw materials.

[0035] (Metallic element M1)

[0036] M1 contained in the present electrolyte is at least one metal element selected from the group consisting of niobium and tantalum.

[0037] When focusing on the valence of the constituent elements of the lithium ion conductive solid electrolyte, M1 is doped as a pentavalent ion, and therefore the valence difference from titanium is 1. Therefore, in order to obtain a neutral charge balance for the entire electrolyte, the number of lithium ions contained in the lithium ion conductive solid electrolyte decreases according to the number x of doped M1 atoms.

[0038] (Crystal Structure)

[0039] The electrolyte preferably has a monoclinic crystal structure that can be confirmed by X-ray diffraction measurement. If the M1 is doped to replace the titanium element, occupying the position of the titanium element and becoming a solid solution in the lithium-containing titanium oxide, it can be confirmed that the basic Li 2 TiO 3 In the X-ray diffraction measurement, it is preferred to confirm that only Li 2 TiO3 The monoclinic crystal structure is not preferred. LiM1O containing doped M1, which is identified as having no ion conductivity or low ion conductivity, is also confirmed. 3 That kind of crystal structure.

[0040] In addition, in the present electrolyte, since the M1 is doped to replace the titanium element, the lattice constant, especially the angle β, changes according to the doping amount of M1, but the monoclinic crystal structure is maintained. However, if the angle β changes according to the doping amount and approaches 90°, it can also become an orthorhombic crystal.

[0041] (Lithium ion conductivity)

[0042] The total lithium ion conductivity σ of this electrolyte at 25°C total Preferably 1.0×10 -6 (S / cm) or more, more preferably 1.5×10 -6 (S / cm) or more, more preferably 2.0×10 -6 The total lithium ion conductivity can be measured by the method described in the examples below.

[0043] (Method for producing lithium ion conductive solid electrolyte)

[0044] The method for producing the present electrolyte is not particularly limited as long as a lithium ion conductive solid electrolyte within the above-mentioned composition range can be obtained. A solid phase reaction, a liquid phase reaction, etc. may be used. The production method using a solid phase reaction is described in detail below.

[0045] Examples of the production method using a solid phase reaction include a production method having at least one mixing step and one calcining step, respectively.

[0046] In the mixing step, compounds containing lithium atoms, titanium atoms, and M1 atoms, respectively, are mixed.

[0047] The compound containing a lithium atom is not particularly limited, but an inorganic compound is preferred from the viewpoint of easy handling. Examples of the inorganic compound containing a lithium atom include lithium carbonate (Li 2 CO 3 ), lithium oxide (Li 2 O) and other lithium compounds. These lithium compounds can be used alone or in combination of two or more. Lithium carbonate (LiCO) is preferably used because it is easy to decompose and react. 2 CO 3 ).

[0048] The compound containing a titanium atom is not particularly limited, and examples thereof include titanium dioxide (TiO 2), tetraethoxytitanium and other titanium compounds. From the perspective of easy handling, inorganic compounds are preferred. These titanium compounds can be used alone or in combination of two or more. From the perspective of cost and easy handling, titanium dioxide (TiO 2 ).

[0049] The compound containing the M1 atom is not particularly limited, but from the viewpoint of easy handling, an inorganic compound is preferred, and compounds such as oxides and nitrates of M1 can be cited. These compounds can be used alone or in combination of two or more. From the viewpoint of cost, oxides are preferably used.

[0050] When M1 is niobium, for example, niobium pentoxide (Nb 2 O 5 ).

[0051] When M1 is tantalum, for example, tantalum pentoxide (Ta 2 O 5 ), Tantalum nitrate (Ta(NO 3 ) 5 ) and other tantalum compounds. These tantalum compounds may be used alone or in combination of two or more. From the perspective of cost, tantalum pentoxide (Ta 2 O 5 )

[0052] The mixing of the above raw materials can be carried out using a roller mill, a ball mill, a small diameter ball mill (bead mill), a medium stirring mill, a jet mill, a mortar, an automatic mixing mortar, a dissolving machine or a jet mill. The ratio of the mixed raw materials is simply a stoichiometric ratio of the composition of the above formula (1). More specifically, in the calcination step described later, since lithium atoms easily flow out of the system, the above-mentioned lithium atom-containing compound can also be added in an excess of about 10% to 20% to adjust.

[0053] The mixing step may be performed in an air atmosphere, more preferably a nitrogen and / or argon gas atmosphere with an adjusted oxygen content.

[0054] In the calcining step, the mixture obtained in the mixing step is calcined. In the case where the calcining step is performed multiple times to form a two-stage process such as low-temperature calcining and high-temperature calcining, a crushing step using a ball mill or a mortar may be provided between the calcining steps in order to crush or reduce the particle size of the primary calcined product.

[0055] The calcining step may be performed in an air atmosphere, more preferably a nitrogen and / or argon gas atmosphere with an adjusted oxygen content.

[0056] As the sintering temperature, the range of 800 to 1200°C is preferred, the range of 850 to 1100°C is more preferred, and the range of 900 to 1000°C is further preferred. If sintered at above 800°C, the solid solution of the metal element M1 is fully carried out and the ion conductivity is improved. If sintered below 1200°C, lithium atoms are not easy to flow out of the system, so it is preferred. The sintering time is preferably 1 to 16 hours, and more preferably 3 to 12 hours. If the sintering time is within the above range, the total lithium ion conductivity tends to increase, so it is preferred. If the sintering time is longer than the above range, lithium atoms tend to flow out of the system. The sintering time and sintering temperature are adjusted in conjunction with each other.

[0057] When the firing step is a two-step process of, for example, low-temperature firing and high-temperature firing, the low-temperature firing may be performed at 400 to 800° C. for 2 to 12 hours.

[0058] In order to suppress the residual by-products, high-temperature calcination may be performed twice. In the second calcination step, the calcination temperature is preferably in the range of 800 to 1200° C., more preferably in the range of 850 to 1100° C., and further preferably in the range of 900 to 1000° C. The calcination time of each calcination step is preferably 1 to 8 hours, more preferably 2 to 6 hours.

[0059] The fired product obtained after firing may absorb moisture and / or react with carbon dioxide to deteriorate if left in the air. The fired product obtained after firing is preferably transferred to a dehumidified inert gas atmosphere and stored when the fired product temperature drops below 200°C during the temperature drop after firing.

[0060] As one of the preferred embodiments of the present electrolyte, use of the electrolyte as a solid electrolyte in a lithium ion secondary battery can be mentioned.

[0061] (Lithium-ion secondary battery)

[0062] One embodiment of the present invention is a lithium ion secondary battery containing the present electrolyte as a solid electrolyte. The structure of the lithium ion secondary battery is not particularly limited. For example, in the case of a solid battery with a solid electrolyte layer, a structure in which a positive electrode collector, a positive electrode layer, a solid electrolyte layer, a negative electrode layer and a negative electrode collector are stacked in sequence is formed.

[0063] The positive electrode current collector and the negative electrode current collector are not particularly limited as long as their materials are materials that conduct electrons without electrochemical reactions. For example, they are composed of a single substance or alloy of a metal such as copper, aluminum, iron, or a conductive metal oxide such as antimony-doped tin oxide (ATO) and tin-doped indium oxide (ITO). Furthermore, a current collector having a conductive adhesive layer on the surface of the conductor can also be used. The conductive adhesive layer can be composed of a granular conductive material and a fibrous conductive material.

[0064] The positive electrode layer and the negative electrode layer can be obtained by a known powder forming method. For example, by overlapping the positive electrode collector, the positive electrode layer powder, the solid electrolyte layer powder, the negative electrode layer powder and the negative electrode collector in sequence, and simultaneously performing powder forming on them, it is possible to form the positive electrode layer, the solid electrolyte layer and the negative electrode layer, and simultaneously perform the connection between the positive electrode collector, the positive electrode layer, the solid electrolyte layer, the negative electrode layer and the negative electrode collector. In addition, each layer can also be powder formed one by one. As needed, the obtained powder formed product can also be subjected to heat treatment such as sintering.

[0065] Examples of powder forming methods include a method comprising adding a solvent to a powder to form a slurry, applying the slurry to a collector, drying it, and then applying pressure (a doctor blade method); a method comprising placing the slurry in a liquid-absorbing mold, drying it, and then applying pressure (a casting method); a method comprising placing the powder in a mold of a predetermined shape for compression molding (a mold forming method); an extrusion forming method comprising extruding the slurry from a mold; a centrifugal force method comprising compressing the powder using centrifugal force; a calendering method comprising supplying the powder to a roller press for calendering; a cold isostatic pressing method comprising placing the powder in a flexible bag of a predetermined shape, placing the bag in a pressure medium, and applying isostatic pressure; a hot isostatic pressing method comprising placing the powder in a container of a predetermined shape to form a vacuum state, and applying isostatic pressure to the container using a pressure medium at a high temperature, and the like.

[0066] As mold forming methods, there can be cited a single-pressing method comprising placing powder in a fixed lower punch and a fixed die, and applying pressure to the powder with a movable upper punch; a double-pressing method comprising placing powder in a fixed die, and applying pressure to the powder with a movable lower punch and a movable upper punch; a floating mold method comprising placing powder in 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; a withdrawal process comprising placing powder in a fixed lower punch and a movable die, moving the movable die while applying pressure to the powder with a movable upper punch, and causing the fixed lower punch to relatively enter the movable die, and the like.

[0067] The thickness of the positive electrode layer is preferably 10 to 200 μm, more preferably 30 to 150 μm, and further preferably 50 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 10 to 200 μm, more preferably 30 to 150 μm, and further preferably 50 to 100 μm.

[0068] (Active material)

[0069] Examples of the active material for the negative electrode include a material containing at least one selected from lithium alloys, metal oxides, graphite, hard carbon, soft carbon, silicon, silicon alloys, silicon oxides SiO n (0 < n ≤ 2), silicon / carbon composites, composites containing silicon domains in the pores of porous carbon, lithium titanate, and graphite covered with lithium titanate. Silicon / carbon composites and composites containing silicon domains in the pores of porous carbon have a high specific capacity and can improve the energy density and battery capacity, and are therefore preferred. More preferably, composites containing silicon domains in the pores of porous carbon are used, which have excellent mitigation of volume expansion accompanying lithium insertion / extraction of silicon and can maintain a good balance of macroscopic conductivity, microscopic conductivity, and ionic conductivity in the composite electrode material or electrode layer. Particularly preferred are composites containing silicon domains in the pores of porous carbon where the silicon domains are amorphous, the size of the silicon domains is 10 nm or less, and pores from the porous carbon are present near the silicon domains.

[0070] Examples of the active material for the positive electrode include a material containing at least one selected from LiCo oxides, LiNiCo oxides, LiNiCoMn oxides, LiNiMn oxides, LiMn oxides, LiMn-based spinels, LiMnNi oxides, LiMnAl oxides, LiMnMg oxides, LiMnCo oxides, LiMnFe oxides, LiMnZn oxides, LiCrNiMn oxides, LiCrMn oxides, lithium titanate, lithium metal phosphates, transition metal oxides, titanium sulfide, graphite, hard carbon, lithium nitrides containing transition metals, silicon oxides, lithium silicates, lithium metal, lithium alloys, Li-containing solid solutions, and lithium-storing intermetallic compounds. LiNiCoMn oxides, LiNiCo oxides, or LiCo oxides are preferred, and LiNiCoMn oxides are more preferred. LiNiCoMn oxides have good affinity with the solid electrolyte and excellent balance of macroscopic conductivity, microscopic conductivity, and ionic conductivity. In addition, LiNiCoMn oxides have a high average potential and can improve the energy density and battery capacity in the balance of specific capacity and stability. In addition, the surface of the active material for the positive electrode can also be covered with this solid electrolyte, lithium niobate, lithium phosphate, or lithium borate.

[0071] The active material in one embodiment of the present invention is preferably granular. The 50% diameter in its volume-based particle size distribution is preferably 0.1 μm or more and 30 μm or less, more preferably 0.3 μm or more and 20 μm or less, further preferably 0.4 μm or more and 10 μm or less, and most preferably 0.5 μm or more and 3 μm or less. In addition, the ratio of the length of the major diameter to the length of the minor diameter (the length of the major diameter / the length of the minor diameter), i.e., the aspect ratio, is preferably less than 3, more preferably less than 2.

[0072] The active material in one embodiment of the present invention may also form secondary particles. In this case, the 50% particle size in the number-based particle size distribution of the primary particles is preferably 0.1 μm or more and 20 μm or less, more preferably 0.3 μm or more and 15 μm or less, further preferably 0.4 μm or more and 10 μm or less, and most preferably 0.5 μm or more and 2 μm or less. In the case of forming an electrode layer by compression molding, the active material is preferably a primary particle. In the case where the active material is a primary particle, even in the case of compression molding, it is not easy to cause damage to the electron conduction path or the hole conduction path.

[0073] Example

[0074] The present invention is specifically described below based on examples. It should be noted that the present invention is not limited to these examples. In addition, the powder X-ray diffraction measurement and ion conductivity evaluation in the examples and comparative examples were performed by the following methods and procedures.

[0075] Embodiment 1:

[0076] (1) Preparation of lithium ion conductive solid electrolyte

[0077] First, titanium dioxide (TiO2) was weighed so that the percentage of niobium atoms to the total number of atoms of titanium and niobium elements was 8%. 2 )(anatase type, manufactured by Fujifilm Wako Pure Chemical Industries, purity 98.5% or more) and niobium pentoxide (Nb 2 O 5 ) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity 99.9%), and lithium carbonate (Li ) was weighed so that the number of lithium atoms was 1.92 times the total number of atoms of titanium and niobium. 2 CO 3 ) (manufactured by Sigma-Aldrich, purity 99.0% or more).

[0078] An appropriate amount of toluene was added to each weighed powder, and the mixture was mixed for 3 hours using a zirconia ball mill (zirconia balls: 1 mm in diameter).

[0079] The obtained mixture was placed in an alumina dish, heated to 700°C at a heating rate of 10°C / min in an air atmosphere (gas flow rate 100 mL / min) using a rotary sintering furnace (produced by Motoyama Co., Ltd.), and sintered at 700°C for 5 hours.

[0080] An appropriate amount of toluene was added to the primary fired product obtained by firing, and the product was crushed for 3 hours using a zirconia ball mill (zirconia balls: 1 mm in diameter).

[0081] The crushed object was placed in an alumina dish, heated to 900°C at a heating rate of 10°C / min in an air atmosphere (gas flow rate 100 mL / min) using a rotary sintering furnace (Motoyama Co., Ltd.), and sintered at 900°C for 3 hours.

[0082] The obtained secondary fired product was cooled, taken out at room temperature, and transferred to a desiccator maintained at a vacuum of 200 Pa or less, thereby obtaining a lithium ion conductive solid electrolyte (1).

[0083] (2) Powder X-ray diffraction (XRD) measurement

[0084] The powder X-ray diffraction measurement of the lithium ion conductive solid electrolyte (1) was performed using a powder X-ray diffraction measurement device Panalytical MPD (manufactured by Spectris Corporation). As the X-ray diffraction measurement conditions, Cu-Kα rays (output power 45 kV, 40 mA) were used to measure in the range of diffraction angle 2θ = 10 to 60 degrees, and the X-ray diffraction (XRD) pattern of the lithium ion conductive solid electrolyte (1) was obtained. The obtained XRD pattern is shown in Figure 1 In the XRD pattern, only the undoped Li 2 TiO 3 The same monoclinic crystal structure.

[0085] (3) Evaluation of ionic conductivity

[0086] (Preparation of measurement pellets)

[0087] The preparation of the measuring pellets for evaluating the ionic conductivity of the lithium ion conductive solid electrolyte is carried out as follows. The obtained lithium ion conductive solid electrolyte (1) is crushed according to the crushing order after the above-mentioned primary calcination, and after being made into powder, it is formed into a disc shape with a diameter of 10 mm and a thickness of 1 mm using a tablet forming machine, and calcined in an atmosphere at 1000°C for 3 hours. The relative density of the obtained calcined product relative to the theoretical density is 93%. On both sides of the obtained calcined product, a gold layer is formed using a sputtering machine to obtain measuring pellets for evaluating the ionic conductivity.

[0088] (Impedance measurement)

[0089] The ionic conductivity of the lithium ion conductive solid electrolyte (1) was evaluated as follows. The measurement pellets prepared by the above method were kept at 25°C for 2 hours before the measurement. Next, an AC impedance measurement was performed at 25°C using an impedance analyzer (Solartron Analytical, model: 1260A) with an amplitude of 25 mV and a frequency range of 1 Hz to 10 MHz. Using the equivalent circuit analysis software ZView included with the device, the obtained impedance spectrum was fitted with an equivalent circuit to obtain the total ionic conductivity. The obtained ionic conductivities are shown in Table 1.

[0090] Comparative Example 1:

[0091] (Preparation of lithium ion conductive solid electrolyte)

[0092] Titanium dioxide and niobium pentoxide were weighed in such a manner that the percentage of niobium atoms relative to the total atomic number of titanium and niobium elements was 16%, and lithium carbonate was weighed in such a manner that the number of lithium atoms was 1.84 times the total atomic number of titanium and niobium elements. A lithium ion conductive solid electrolyte (2) was obtained in the same manner as in Example 1, except that the above-mentioned amounts were not less than 1.5%.

[0093] (XRD measurement, ion conductivity evaluation)

[0094] XRD measurement and ion conductivity evaluation were performed in the same manner as in Example 1. The XRD pattern of the lithium ion conductive solid electrolyte (2) is shown in FIG. Figure 1 In the XRD pattern of the lithium ion conductive solid electrolyte (2), except for the region identified as Li 2 TiO 3 In addition to the diffraction peaks derived from the monoclinic crystal structure (ICSD reference code: 15150), the diffraction peaks derived from lithium niobate (LiNbO 3 ) has a diffraction peak of a trigonal crystal structure (ICSD reference code: 74469). More specifically, in the diffraction pattern of the lithium ion conductive solid electrolyte (2) of Comparative Example 1, LiNbO 3 On the other hand, these peaks were not observed in the diffraction pattern of the lithium ion conductive solid electrolyte (1) of Example 1. Therefore, it can be confirmed that the lithium ion conductive solid electrolyte (1) of Example 1 is only identified as Li 2 TiO 3 The monoclinic crystal structure.

[0095] Table 1 also shows the total ion conductivity of the lithium ion conductive solid electrolyte (2).

[0096] Comparative Example 2:

[0097] (Preparation of lithium ion conductive solid electrolyte)

[0098] A lithium ion conductive solid electrolyte (3) was obtained in the same manner as in Example 1 except that niobium pentoxide was not used and titanium dioxide and lithium carbonate in an amount such that the number of lithium atoms was 2.00 times the number of titanium atoms contained in titanium dioxide were weighed.

[0099] (XRD measurement, ion conductivity evaluation)

[0100] XRD measurement and ion conductivity evaluation were performed in the same manner as in Example 1. The obtained XRD pattern is shown in FIG. Figure 1 In the XRD pattern of the lithium ion conductive solid electrolyte (3), only the Li 2 TiO 3 The monoclinic crystal structure.

[0101] The ion conductivity of the lithium ion conductive solid electrolyte (3) was too low to be measured.

[0102] Table 1

[0103]

[0104] Examples 2 to 3 and Comparative Examples 3 to 5:

[0105] (Theoretical calculation)

[0106] The electrolyte is composed of Li 2 TiO 3 In the lithium-containing titanium oxide represented by the electrolyte, the M1 element doped with titanium is partially replaced. In addition, as described above, M1 is doped, replaces the titanium element and occupies the position of the titanium element, and becomes a solid solution in the lithium-containing titanium oxide, maintaining the basic Li 2 TiO 3 Here, using theoretical calculations, in Li 2 TiO 3 In this work, a search was conducted for M1 elements that could be substituted in the titanium position and dissolved in the solid state.

[0107] Specifically, prepare 2 TiO 3 The most stable structures of the supercell in which Ti atoms are replaced by M1 atoms were obtained using first-principles calculations and compared.

[0108] The first-principles calculations were performed as follows. A supercell (Li 32 Ti 16 O48 ) is replaced by an M1 atom. When the M1 atom is Nb, Ta or V, the electrical neutrality condition is considered. As a structure in which one Li atom is removed, a supercell of all atomic configurations with different geometric symmetries is prepared. In this structure, the doping amount of the M1 element is equivalent to 6%. The atomic configuration of the above supercell is input into the first-principles calculation package software Vienna Ab initio Simulation Package (VASP) (processed by HPC Systems Co., Ltd.), and M1 is replaced by Nb, Ta, V, Sn and Si. The structure is optimized with the composition of each example shown in Table 2, and the energy calculation is performed respectively.

[0109] The structural optimization based on the first-principles calculation was performed under the following conditions.

[0110] ·Pseudopotential: Projector Augmented Wave (PAW) method

[0111] Exchange-correlation functional: Generalized Gradient Approximation (GGA)

[0112] Energy cutoff: 520eV

[0113] k-point grid: 2×2×2

[0114] The energy values ​​of all compositions of Li, Ti, M1, and O, including single, binary, ternary, and quaternary systems, were calculated using the Materials Project database provided by Kristin Persson et al. (website "https: / / materialsproject.org / ", accessed in August 2021). The Li partially replaced by the M1 element 2 TiO 3 The energy increment per atom from the convex hull (meV / atom) is shown in Table 2. Regarding the difference in geometric symmetry, the atomic configuration with the lowest geometric symmetry among the calculated energies was adopted. It is believed that the smaller the energy increment from the convex hull, the more Li 2 TiO 3 The more stable.

[0115] Table 2

[0116] Composition Energy difference between each atom and the convex hull (meV / atom) Example 2 <![CDATA[Li 1.94 Of 0.84 Nb 0.06 SHE 3 ]]> 8.7 Example 3 <![CDATA[Li 1.94 Ti 0.94 The 0.06 The 3 ]]> 8.4 Comparative Example 3 <![CDATA[Li 1.94 You 0.94 V 0.06 About 3 ]]> 15.7 Comparative Example 4 <![CDATA[Li 2.00 Of 0.94 Sn 0.06 SHE 3 ]]> 10.3 Comparative Example 5 <![CDATA[Li 2.00 Of 0.94 To 0.06 SHE 3 ]]> 12.8

[0117] The calculation results shown in Table 2 show that, compared with V, Sn and Si in Comparative Examples 3 to 5, when Nb and Ta as M1 shown in Examples 2 and 3 are used, 6% of Li is replaced by M1 elements. 2 TiO 3 is stable.

[0118] Comparative Examples 6 to 7:

[0119] (Preparation of lithium ion conductive solid electrolyte)

[0120] Except for using tin (IV) oxide (manufactured by Sigma-Aldrich, purity 99.0% or more) or silicon oxide (manufactured by Sigma-Aldrich, purity 99.0% or more) instead of niobium pentoxide, the lithium ion conductive solid electrolyte (4) of Comparative Example 6 and the lithium ion conductive solid electrolyte (5) of Comparative Example 7 were obtained in the same manner as in Example 1.

[0121] (XRD measurement)

[0122] XRD measurement was performed in the same manner as in Example 1. The crystal structures confirmed in the XRD measurement are shown in Table 3 together with the results of Example 1 (ICSD reference code, Li 2 SnO 3 :21032、Li 2 SiO 3 :100402).

[0123] Table 3

[0124] Composition Confirmed crystal structure Example 1 <![CDATA[Li 1.92 Of 0.92 Nb 0.08 SHE 3 ]]> <![CDATA[Li 2 TiO 3 ]]> Comparative Example 6 <![CDATA[Li 2.00 Of 0.92 Sn 0.08 SHE 3 ]]> <![CDATA[Li 2 TiO 3 、Li 2 SnO 3 ]]> Comparative Example 7 <![CDATA[Li 2.00 Of 0.92 To 0.08 SHE 3 ]]> <![CDATA[Li 2 TiO 3 、Li 2 SiO 3 ,

[0125] The results shown in Table 3 confirm that in the lithium ion conductive solid electrolytes (4) and (5) of Comparative Examples 6 and 7, in the actually produced lithium ion conductive solid electrolytes, the M1 atoms are stable and not dissolved, and Li 2 TiO 3 The experimental results confirm the above theoretical calculations.

[0126] The results of the examples confirm that the ion conductivity of the lithium ion conductive solid electrolyte having the composition formula Li 2-x Ti 1-x M1 x O 3 A compound represented by: M1 is at least one metal element selected from the group consisting of niobium and tantalum, and 0.05≤x≤0.15.

[0127] Industrial Applicability

[0128] The lithium ion conductive solid electrolyte of the present invention is an oxide-based lithium ion conductive solid electrolyte having high ion conductivity, and can be suitably used as a solid electrolyte for lithium ion secondary batteries.

Claims

1. A lithium ion conductive solid electrolyte, It is composed of the formula Li 2-x Ti 1-x M1 x O 3 The compound represented by The M1 is at least one metal element selected from the group consisting of niobium and tantalum, 0.05≤x≤0.15。 2. The lithium ion conductive solid electrolyte according to claim 1, wherein a monoclinic crystal structure can be confirmed in X-ray diffraction measurement.

3. The lithium ion conductive solid electrolyte according to claim 1 or 2, wherein the total lithium ion conductivity σ at 25°C is total 1.0×10 -6 S / cm or more. 4 . A lithium ion secondary battery comprising the lithium ion conductive solid electrolyte according to claim 1 as a solid electrolyte.

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