Oxide and method for producing same, oxide dispersion liquid, solid electrolyte, and power storage device

Abstract

This oxide contains Na, Zr, Si, and P as constituent elements, has a molar ratio of Zr to Si of 0.60 to 0.87 inclusive, and additionally contains a specific element that has an electronegativity of 1.70 or more and is different from Si and P.

Description

Oxides and methods for producing the same, oxide dispersions, solid electrolytes, and energy storage devices

[0001] [Cross-reference of related applications] This application claims priority under Japanese Patent Application No. 2025-46139, filed on March 20, 2025, which is incorporated herein by reference in its entirety. This disclosure relates to oxides and methods for producing the same, oxide dispersions, solid electrolytes, and energy storage devices.

[0002] Various energy storage devices have been put into practical use, including nickel-metal hydride rechargeable batteries, lithium-ion rechargeable batteries, and electric double-layer capacitors. Among these, lithium-ion rechargeable batteries (LIBs) are used in a wide range of applications due to their high energy density and battery capacity.

[0003] Lithium-ion batteries (LIBs), widely used as energy storage devices, are secondary batteries that have a negative electrode, a positive electrode, and an electrolyte, and charge and discharge by moving lithium ions between the two electrodes via the electrolyte. Conventionally, non-aqueous electrolytes have been mainly used as the electrolyte. However, since non-aqueous electrolytes contain flammable organic solvents, there are concerns about electrolyte leakage and short circuits inside the battery due to overcharging and over-discharging. In light of these points, all-solid-state lithium-ion secondary batteries using a solid electrolyte with lithium-ion conductivity instead of a non-aqueous electrolyte are being investigated.

[0004] On the other hand, lithium, the raw material for LIBs, is facing concerns due to soaring raw material prices and depletion issues. Therefore, in recent years, various studies have been conducted on sodium-ion secondary batteries (SIBs) that use sodium, which is abundant and inexpensive, as a post-LIB alternative to lithium, a rare metal, and charge and discharge by the movement of sodium ions (see, for example, Patent Document 1).

[0005] Solid electrolytes with sodium ion conductivity include sulfide-based solid electrolytes and oxide-based solid electrolytes. Of these, oxide-based solid electrolytes are generally considered to have high stability in the atmosphere and excellent safety. One example of an oxide-based solid electrolyte exhibiting sodium ion conductivity is a NaSICON-type crystal structure. ３ Zr ２ Si ２ PO １２ It is known that various methods have been investigated to improve ionic conductivity by doping with various elements (see, for example, Non-Patent Document 1). Non-Patent Document 1 mentions Na ３ Zr ２ Si ２ PO １２ It has been disclosed that doping with cerium (Ce) can enhance the Na ion conductivity.

[0006] International Publication No. 2019 / 003846

[0007] ACS APPLIED MATERIALS & INTERFACES, 2020, No. 12, PP. 3502-3509

[0008] To achieve higher capacity and higher output in all-solid-state solid-state batteries (SIBs), it is necessary to develop solid electrolytes that exhibit high sodium ion conductivity. However, the oxide-based solid electrolyte described in Non-Patent Document 1 has insufficient sodium ion conductivity, and further improvements are needed.

[0009] The inventors investigated the oxide-based solid electrolyte described in Non-Patent Literature 1 and found that it sometimes warped during the fabrication of the sintered body. Furthermore, when fabricating an energy storage device equipped with a solid electrolyte by co-firing the oxide-based solid electrolyte with secondary battery materials such as active materials, if the oxide-based solid electrolyte warps, there is a risk that the bonding at the interface between the electrode and the electrolyte will deteriorate. In addition, if the bonding at the interface between the electrode and the electrolyte deteriorates, it is a concern that the overall resistance of the cell may increase or cracks may occur inside the cell material, making it difficult to operate as an energy storage device.

[0010] This disclosure has been made in view of the above circumstances, and its purpose is to provide an oxide that exhibits high Na ion conductivity and is less prone to bending during the fabrication of sintered bodies.

[0011] As a result of diligent research to solve the above problems, the inventors have found that the above problems can be solved by doping an oxide containing Na, Zr, Si, and P as constituent elements with an element that has high electronegativity, and by keeping the molar ratio of Zr to Si within a predetermined range. According to this disclosure, the following oxides, methods for producing the same, oxide dispersions, solid electrolytes, and energy storage devices are provided.

[0012] [1] An oxide comprising Na, Zr, Si, and P as constituent elements, wherein the molar ratio of Zr to Si is 0.60 or more and 0.87 or less, and further comprises a specific element that is different from Si and P and has an electronegativity of 1.70 or more. [2] The oxide according to [1], wherein the electronegativity of the specific element is 2.60 or less. [3] The oxide according to [1] or [2], satisfying the following general formula (1). (In general formula (1), M1 contains an element that forms a divalent cation, M2 contains an element that forms a trivalent cation, M3 contains an element that forms a tetravalent cation excluding Zr and Si, M4 contains an element that forms a pentavalent cation excluding P, and M5 contains an element that forms a hexavalent cation, and one or more of M1, M2, M3, M4 and M5 contains the specified element. a, b, c, d and e are each independently 0 or greater, satisfying "0 < a + b + c + d + e < 2", "2a + b - d - 2e + α > 0" and "0 < α < 1".) [4] An oxide according to any one of [1] to [3] having a NASICON-type crystal structure. [5] A solid electrolyte containing an oxide according to any one of [1] to [4]. [6] The solid electrolyte according to [5] having a relative density of 80% or more. [7] An energy storage device comprising the solid electrolyte described in [5] or [6]. [8] An oxide dispersion containing the oxide described in any of [1] to [4] and a solvent. [9] A method for producing the oxide described in [3], comprising the steps of: mixing a supply component containing at least one selected from the group consisting of Na, Zr, Si, P and the specified element in such a way that it satisfies the stoichiometric ratio represented by the general formula (1) above to obtain a mixture of the supply component; and calcining the mixture to obtain the oxide.

[10] The method for producing the oxide described in [9], wherein the mixing of the supply component is wet mixing.

[11] The method for producing the oxide described in [9] or

[10] , wherein the mixture is calcined at 800°C or higher.

[0013] According to this disclosure, an oxide exhibiting high Na ion conductivity and less prone to bending during the fabrication of sintered bodies can be obtained. Furthermore, by using the oxide of this disclosure as an electrolyte material for energy storage devices such as secondary batteries and capacitors, it is possible to obtain an energy storage device that combines safety assurance through the solidification of the electrolyte with high performance due to high Na ion conductivity.

[0014] The oxides (hereinafter also referred to as "Oxide (X)"), oxide dispersions, solid electrolytes, and energy storage devices described herein will be explained in detail below.

[0015] ≪Oxide (X)≫ Oxide (X) contains Na, Zr, Si and P as constituent elements, has a molar ratio of Zr to Si of 0.60 or more and 0.87 or less, and further contains an element which has an electronegativity of 1.70 or more and is different from the above-mentioned constituent elements (Na, Zr, Si and P). In the present specification, "an element which has an electronegativity of 1.70 or more and is different from Si and P" is also referred to as a "specific element".

[0016] Examples of the oxide (X) include oxides obtained by substituting a part of elements constituting Na ３ Zr ２ Si ２ PO １２ (also referred to as "NZSP"), which is a NASICON-type solid electrolyte, with various elements containing at least the specific element.

[0017] The specific element is an element having an electronegativity of 1.70 or more, and is different from Si and P. Note that the electronegativity in the present specification represents Pauling electronegativity. In Pauling electronegativity, the electronegativity of Na, Zr, Si and P is 0.93, 1.33, 1.90 and 2.19, respectively. In an oxide containing Na, Zr, Si and P as constituent elements, doping with an element having an electronegativity of 1.70 or more makes it possible to obtain an oxide that exhibits high Na ion conductivity and is less prone to warping during production of a sintered body. Only one type of the specific element to be doped may be used, or two or more types may be used.

[0018] From the viewpoint of obtaining an oxide with better Na ion conductivity, the electronegativity of the specific element is preferably 1.73 or more, more preferably 1.75 or more. Further, from the viewpoint of ease of doping, the electronegativity of the specific element is preferably 2.60 or less, more preferably 2.58 or less, still more preferably 2.55 or less, even more preferably 2.50 or less, and still even more preferably 2.40 or less.

[0019] The preferred range for the electronegativity of a specific element can be set by appropriately combining the preferred upper and lower limits mentioned above. The electronegativity of a specific element is preferably 1.70 to 2.60, more preferably 1.73 to 2.58, even more preferably 1.73 to 2.55, even more preferably 1.75 to 2.55, and even more preferably 1.75 to 2.50.

[0020] As for specific elements, elements from Groups 6 to 17 with an electronegativity of 1.70 or higher (excluding O, Si, and P) can be preferably used in terms of ease of doping. Further specific examples of specific elements include Mo, W, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Hg, B, Ga, In, C, Ge, Sn, Pb, N, As, Sb, Bi, S, Se, Se, Te, Po, F, Cl, Br, I, At, etc. Among these, elements from Groups 6 to 16 are preferred in that oxides with superior Na ion conductivity can be obtained, and elements from the 3rd to 6th periods of Groups 6 to 16 are more preferred. Among these, at least one selected from the group consisting of Ga, Ge, Fe, In, Sn, Pb, Co, Ni, Cu, Mo, Ag, Te, Sb, Bi, and W is particularly preferred.

[0021] When obtaining an oxide (X) by substituting some of the constituent elements of NZSP, the elements used for substitution (i.e., doping elements) may consist only of specific elements, or they may also contain elements other than the specific elements (hereinafter also referred to as "other elements"). Examples of other elements include Al, Mg, Ca, Ba, Sc, Y, Sr, Ti, Hf, V, Cr, Mn, Zn, Nb, Tl, Ta, and lanthanide elements. When other elements are used as doping elements, there may be only one type of other element, or there may be two or more types.

[0022] The oxide (X) has a molar ratio of Zr to Si (hereinafter also referred to as "Zr / Si") of 0.60 or more and 0.87 or less. If the Zr / Si is less than 0.60 or greater than 0.87, the resulting oxide will have insufficient Na ion conductivity and will be prone to warping during the fabrication of the sintered body. From the viewpoint of obtaining an oxide with excellent Na ion conductivity and that is less prone to warping during the fabrication of the sintered body, the Zr / Si of oxide (X) is preferably 0.65 or more, more preferably 0.68 or more, and even more preferably 0.70 or more. The Zr / Si of oxide (X) is a value obtained by elemental composition analysis by radio frequency inductively coupled plasma (ICP) emission spectrometry.

[0023] The preferred range for the Zr / Si ratio of oxide (X) can be determined by appropriately combining the lower and upper limits mentioned above. The preferred range for the Zr / Si ratio of oxide (X) is 0.65 to 0.87, more preferably 0.68 to 0.87, and even more preferably 0.70 to 0.87.

[0024] The melting point of oxide (X) is, for example, 1,050°C or higher, but may also be 1,100°C or higher, 1,150°C or higher, 1,200°C or higher, or 1,250°C or higher. There is no particular upper limit to the melting point of oxide (X). Note that the melting point of oxide (X) is the value at atmospheric pressure.

[0025] The phase structure of oxide (X) is not particularly limited. It is preferable that oxide (X) has a NASICON-type crystal structure, as this allows for the acquisition of a solid electrolyte with high Na ion conductivity. Unlike layered structures, the NASICON-type crystal structure provides a three-dimensionally expanded space for alkali metal ion movement, and zirconium (Zr) remains stable even under high voltages, making it useful as a solid electrolyte for high operating voltages. The crystal structure of oxide (X) can be determined from the diffraction profile obtained by powder X-ray diffraction measurement.

[0026] Because of its excellent Na ion conductivity, the oxide (X) is preferably satisfied with the following general formula (1). (In general formula (1), M1 contains an element that forms a divalent cation, M2 contains an element that forms a trivalent cation, M3 contains an element that forms a tetravalent cation (excluding Zr and Si), M4 contains an element that forms a pentavalent cation (excluding P), and M5 contains an element that forms a hexavalent cation. One or more of M1, M2, M3, M4, and M5 contains a specific element. a, b, c, d, and e are each independently greater than or equal to 0, satisfying "0 < a + b + c + d + e < 2", "2a + b - d - 2e + α > 0", and "0 < α < 1".)

[0027] Substances that satisfy the above general formula (1) are Na ３ Zr ２ Si ２ PO １２ In an oxide with (NZSP) as the basic framework, a portion of Zr is substituted with at least one of the doping elements M1 (containing an element that forms a divalent cation), M2 (containing an element that forms a trivalent cation), M3 (containing an element that forms a tetravalent cation other than Zr and Si), M4 (containing an element that forms a pentavalent cation other than P), and M5 (containing an element that forms a hexavalent cation), and the oxide has a crystal structure. However, a substance that satisfies the above general formula (1) contains at least one specific element as a doping element, and a portion of Zr is substituted with at least one specific element.

[0028] In the general formula (1) above, M1 can be any element from Group 2, Group 12, or transition elements (Groups 3 to 11) that form a divalent cation. Further specific examples of M1 include Mg, Ca, Sr, Ba, Zn, Cu, and Ni.

[0029] Examples of M2 elements include those from Group 3, Group 13, Group 15, and transition elements (Groups 3-11) that form trivalent cations. Further specific examples of M2 elements include Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, B (boron), Al, Ga, In, Sb, and Bi.

[0030] M3 elements are those that form tetravalent cations other than Zr and Si. Examples of M3 elements include Group 14 elements other than Si (Ge, Sn, Pb), Group 16 elements that form tetravalent cations (Se, Te), and transition elements other than Zr (Groups 3-11 elements) that form tetravalent cations (Ti, Hf, Ce, Pr, Tb).

[0031] Examples of M4 elements include those from Group 15 and transition elements (Groups 3-11) that form pentavalent cations. Further specific examples of M4 elements include V, Nb, Ta, Sb, and Bi. Examples of M5 elements include those from Group 16 and transition elements (Groups 3-11) that form hexavalent cations. Further specific examples of M5 elements include Te, Mo, and W.

[0032] The specified elements may consist of only one of M1, M2, M3, M4, and M5, or two or more. Specific examples of the specified elements include Cu as M1; In, Ga, and Bi as M2; Ge, Te, and Sn as M3; Sb as M4; and W (tungsten) as M5.

[0033] In the above general formula (1), a, b, c, d, e, and α are not particularly limited, as long as a, b, c, d, and e are each greater than or equal to 0 and satisfy the conditions "0 < a + b + c + d + e < 2", "2a + b - d - 2e + α > 0", and "0 < α < 1". For example, if a = 0, b = 0, c > 0, d > 0 and e = 0, the substance is "Na ３－ｄ＋α M3 ｃ M4 ｄ Zr ２－ｃ－ｄ Si ２＋α P １－α O １２ It is represented by ". If a=0, b>0, c>0, d=0 and e=0, then the substance is "Na ３＋ｂ＋α M2 ｂ M3 ｃ Zr ２－ｂ－ｃ Si ２＋α P １－α O １２ This is represented by ".

[0034] More specifically regarding a, b, c, d, e, and α, it is preferable that a ≤ 0.3 is satisfied, more preferably a ≤ 0.2, and even more preferably a ≤ 0.1, in that impurity phases are less likely to form, thereby enabling the acquisition of a solid electrolyte exhibiting high Na ion conductivity. When a > 0, the lower limit of a is preferably a ≥ 0.01, and more preferably a ≥ 0.03.

[0035] Regarding b, if b > 0, it is preferable that b ≥ 0.01 is satisfied, more preferably b ≥ 0.02, even more preferably b ≥ 0.03, and even more preferably b ≥ 0.04, in which case a solid electrolyte exhibiting high Na ion conductivity can be obtained. Regarding the upper limit of b, it is preferable that b ≤ 0.5 is satisfied, more preferably b ≤ 0.4, even more preferably b ≤ 0.3, even more preferably b ≤ 0.2, and even more preferably b ≤ 0.1.

[0036] With respect to c, it is preferable that c ≤ 0.3, more preferably c ≤ 0.2, and even more preferably c ≤ 0.1, as this suppresses the formation of impurity phases and thereby allows for the acquisition of a solid electrolyte with higher Na ion conductivity. Furthermore, when c > 0, the lower limit of c is preferably c ≥ 0.01, and more preferably c ≥ 0.03.

[0037] With respect to d, it is preferable that d ≤ 0.3 is satisfied, more preferably that d ≤ 0.2 is satisfied, and even more preferably that d ≤ 0.1 is satisfied, in order to suppress the formation of impurity phases and obtain a solid electrolyte that exhibits higher Na ion conductivity. When d > 0, the lower limit of d is preferably d ≥ 0.01, and more preferably that d ≥ 0.03 is satisfied.

[0038] With respect to e, it is preferable that e ≤ 0.3, more preferably e ≤ 0.2, and even more preferably e ≤ 0.1, in that it suppresses the formation of impurity phases and allows for the acquisition of a solid electrolyte exhibiting higher Na ion conductivity. When e > 0, the lower limit of e is preferably e ≥ 0.01, and more preferably e ≥ 0.03.

[0039] It is preferable that α satisfies 0.1 ≤ α ≤ 0.8. Regarding the upper limit of α, since excessive Si substitution can lead to a decrease in Na ion conductivity, it is more preferable that α ≤ 0.7, even more preferable that α ≤ 0.6, and even more preferable that α ≤ 0.5. Furthermore, regarding the lower limit of α, it is more preferable that α ≥ 0.15, even more preferable that α ≥ 0.2, even more preferable that α ≥ 0.25, and even more preferable that α ≥ 0.3, in order to obtain a solid electrolyte exhibiting higher Na ion conductivity.

[0040] In general formula (1) above, the stoichiometric ratio of O is given as 12, but it is sufficient to maintain the charge neutrality of the oxide (X) as a whole, and the stoichiometric ratio of O does not have to be exactly 12. In other words, the amount of O derived from the oxide (X) may be less than 12 or greater than 12, as long as the charge neutrality of the oxide (X) as a whole is maintained.

[0041] The method for producing an oxide that satisfies the above general formula (1) is not particularly limited. The oxide (X) is preferably produced by a method comprising the following mixing step and calcination step. Mixing step: A step of mixing supply components, which include at least one selected from the group consisting of Na, Zr, Si, P, and specific elements, in such a way that satisfies the stoichiometric ratio represented by the above general formula (1) to obtain a mixture of supply components. Calcination step: A step of calcining the mixture obtained in the mixing step to obtain an oxide.

[0042] (Mixing process) As raw materials for oxide (X), the following can be used: Na supplying component, M1 supplying component, M2 supplying component, M3 supplying component, M4 supplying component, M5 supplying component, Zr supplying component, Si supplying component, and P supplying component, with the supplying component corresponding to the element required to obtain the desired oxide (X). For example, when producing an oxide in which a=0, b=0, c>0, d>0 and e=0 in the above general formula (1), Na supplying component, M3 supplying component, M4 supplying component, Zr supplying component, Si supplying component, and P supplying component are used as raw materials for oxide (X). These supplying components may be compounds in which one type of supplying component contains two or more elements from among Na, M1, M2, M3, M4, M5, Zr, Si, and P.

[0043] The raw materials for oxide (X), namely the Na supply component, M1 supply component, M2 supply component, M3 supply component, M4 supply component, M5 supply component, Zr supply component, and Si supply component, may be inorganic compounds or organic compounds. For each supply component, for example, carbonates, bicarbonates, sulfates, sulfites, nitrates, nitrites, phosphates, acetates, citrates, ammonium salts, oxides, hydroxides, chlorides, sulfides, etc. of these metal elements can be used. As the P supply component, phosphates can be preferably used, and among these, sodium phosphate or ammonium dihydrogen phosphate are preferred.

[0044] In the production of oxide (X), the mixing of the supply components in the mixing process may be carried out by dry mixing or by wet mixing using a liquid. Of these, wet mixing is preferred. By employing wet mixing, the supply components can be thoroughly mixed, and the density of oxide (X) can be increased compared to the case of dry mixing. This also relatively improves the Na ion conductivity of oxide (X). As the liquid used for wet mixing, water, various organic solvents, and mixtures thereof can be used as appropriate. The slurry obtained by wet mixing may have coarse particles or aggregates removed using, for example, a sieve. Furthermore, the mixing of the supply components may be carried out at room temperature, or at low or high temperatures.

[0045] (Casturing process) Next, the mixture obtained in the mixing process is calcined. The calcination of the mixture may be carried out in one step or in two or more steps. The calcination temperature is not particularly limited, but from the viewpoint of obtaining an oxide that exhibits good Na ion conductivity, 800°C or higher is preferred, 850°C or higher is more preferred, 900°C or higher is even more preferred, 950°C or higher is even more preferred, and 1,000°C or higher is even more preferred.

[0046] When a mixture is subjected to multiple firing processes, the firing process may include a pre-firing process in which the mixture is fired at a pre-firing temperature, and a final firing process in which the mixture is fired at a final firing temperature higher than the pre-firing temperature.

[0047] ・Calibration process The calcination temperature (calcination temperature) in the calcination process is preferably 800°C or higher, more preferably 900°C or higher, and even more preferably 1,000°C or higher. The upper limit of the calcination temperature can be, for example, 1,500°C or lower, preferably 1,400°C or lower, more preferably 1,350°C or lower, even more preferably 1,300°C or lower, and even more preferably 1,250°C or lower. During calcination, the temperature may be gradually increased from a lower temperature than the calcination temperature, and finally maintained at the calcination temperature. The calcination time is not particularly limited, but may be, for example, 1 to 72 hours, or 3 to 48 hours.

[0048] The oxide obtained by the calcination process (hereinafter also referred to as "calcined product") may be pulverized by any method before being subjected to the calcination process in the main calcination step. The calcined product can be pulverized using various pulverizers such as ball mills, planetary ball mills, bead mills, jet mills, blenders, roll crushers, hammer mills, pin mills, and mortars. The pulverization process may also be carried out with the addition of a liquid. As the liquid, water, various organic solvents, and mixtures of two or more of water and various organic solvents can be used as appropriate.

[0049] ・Main firing process In the main firing process, the calcined product obtained in the calcination process may be fired without molding, or it may be fired after molding. The molding method is not particularly limited, and known methods such as extrusion molding, injection molding, pressure molding, slip molding, mold casting, and tape molding can be used. Of these, it is preferable to obtain the molded body by pressure molding because it can minimize the porosity of the fired product (also called a sintered body) obtained by the main firing process and facilitates the formation of ion conduction paths.

[0050] In the main firing process, the composition containing the oxide obtained in the calcination process and other components may be molded as needed before the firing process. For example, a sintering aid may be used as the other component. Examples of sintering aids include oxides and carbonates. Further specific examples of sintering aids include Na ２ O, Na ２ O ２ NaHCO ３ Na ２ CO ３ , NaF, NaCl, NaBr, NaI, NaNO ３ Na ２ SO ３ Na ２ SO ４ , B ２ O ３ H ３ BO ３ NaBH ４ Na ２ B ４ O ７ Na ２ B ４ O ７ 10H ２ O, Na ３ BO ３ , Bi ２ O ３ , Bi ２ O ５ BiF ３ BiCl ３ BiBr ３ , BiOCl, Bi(OH) ３ ZnO, ZnCl ３ , Zn(OH) ２ , Zn (NO ３ ) ２, Zn(NO ３ ) ２ ·6H ２ O, ZnSO ４ , ZnSO ４ ·7H ２ O, P ２ O ５ , Na ２ HPO ４ , Na ２ HPO ４ ·12H ２ O, NaH ２ PO ４ , NaH ２ PO ４ ·2H ２ , Na ３ PO ４ , Na ３ PO ４ ·12H ２ O, 60Na ２ O-10Nb ２ O ５ -30P ２ O ５ and the like. When a sintering aid is used, the content of the sintering aid is preferably 20% by mass or less, more preferably 15% by mass or less, and still more preferably 10% by mass or less, based on the total amount of the calcined product and other components.

[0051] In addition to sintering aids, examples of other components include polymer solid electrolytes, polymer gel electrolytes, inorganic fillers, conductive aids, plasticizers, binders, and the like. However, considering the Na ion conductivity of the sintered body obtained by firing the raw material of oxide (X) and the ease of production, it is preferable that the content of components other than the sintering aid is as low as possible. Specifically, the content of components other than the sintering aid is preferably 1% by mass or less, more preferably 0.5% by mass or less, and still more preferably 0.1% by mass or less, based on the total amount of the calcined product and other components.

[0052] The firing temperature (main firing temperature) when firing the calcined product (main firing) is preferably 850°C or higher, more preferably 900°C or higher, still more preferably 950°C or higher, even more preferably 1,000°C or higher, still even more preferably 1,100°C or higher, and particularly still more preferably 1,200°C or higher. This is because such a temperature can sufficiently reduce interface resistance in the sintered body obtained by main firing, and increase Na ion conductivity. Further, regarding the upper limit of the main firing temperature, it is preferably 1,600°C or lower, and more preferably 1,500°C or lower. The firing time is, for example, 1 to 72 hours, and may be 3 to 48 hours. During main firing, the temperature may be increased stepwise from a temperature lower than the main firing temperature (pre-firing), and finally the temperature required for firing may be maintained. Further, for the purpose of strain removal or the like, a treatment of heating the sintered body at a temperature lower than the main firing temperature may be performed after main firing.

[0053] The oxide (X) as a sintered body obtained in this manner exhibits excellent Na ion conductivity and has a small degree of warpage accompanying firing. Such an oxide (X) is useful as a solid electrolyte for electricity storage devices in which the ion-conducting carriers are sodium ions.

[0054] Specifically, for the oxide (X), the ionic conductivity measured at 25°C using the alternating current impedance method is, for example, 5.0×10 －４ S / cm or higher. From the viewpoint of obtaining an electricity storage device with excellent performance, the ionic conductivity under the same conditions is 1.0×10 －３ S / cm or higher, preferably 1.5×10 －３ S / cm or higher, more preferably. Further, the intragranular ionic conductivity is 2.0×10 －３ S / cm or higher, preferably 2.5×10 －３ S / cm or higher, more preferably. The details of the measurement methods for ionic conductivity and intragranular ionic conductivity follow the methods described in the examples below.

[0055] The relative density of the oxide (X) is preferably 75% or more, more preferably 80% or more, and even more preferably 85% or more, in order to obtain a solid electrolyte material that exhibits excellent Na ion conductivity. In this specification, "relative density" represents the ratio (%) of the measured density to the theoretical density. The measured density is obtained by measuring the diameter, thickness, and mass of the oxide, and using the volume obtained from the measured diameter and thickness, respectively, and the value calculated from the measured mass.

[0056] Here, the following are possible reasons, although not limiting to the present invention, for the oxide (X) exhibiting high Na ion conductivity and for the sintered body being less prone to warping during fabrication. It is thought that by doping an oxide containing Na, Zr, Si, and P as constituent elements with a specific element having an electronegativity of 1.70 or higher, the difference in electronegativity between the atom to which the pre-doped atom (typically a Zr atom) was bonded and the O atom became smaller, thereby reducing the polarizability of the bond. As a result, in oxide (X), the constraint of Na ions by the O atom weakened, and it is presumed that Na ion conduction became easier. Furthermore, it is presumed that the covalent bonding of the entire crystal improved due to doping with the specific element, and as a result of reduced compositional unevenness due to the volatilization of P atoms, etc., uniform sintering could be carried out, which is presumed to have reduced the warping of the sintered body during fabrication. In addition, it is thought that the fact that the molar ratio of Zr to Si (Zr / Si) was within a specific range contributed to the sintered body being less prone to warping during fabrication and to the high Na ion conductivity of the resulting sintered body.

[0057] ≪Oxide Dispersion≫ The oxide dispersion of this disclosure contains the oxide (X) described above and a solvent. The dispersion in which the oxide (X) is dispersed in the solvent is useful, for example, as a coating material for energy storage devices in which the ion carrier is sodium ions. Specifically, by coating the components and parts that make up the energy storage device with the oxide dispersion of this disclosure, it is possible to protect the coated object and suppress the degradation of the performance of the energy storage device, or to improve the performance of the energy storage device. When the oxide dispersion of this disclosure is used as a coating material for energy storage devices, examples of objects to be coated include active materials used as electrode materials (positive electrode active material, negative electrode active material), and separators provided in energy storage devices in which the electrolyte is liquid.

[0058] When preparing an oxide dispersion, water, an organic solvent, or a mixture of water and an organic solvent can be appropriately selected and used as the solvent. Specific examples of organic solvents include alcohols, ethers, ketones, nitriles, esters, amides, amines, and hydrocarbons. Of these, at least one selected from the group consisting of water, alcohols, amides, ketones, and hydrocarbons is preferably used as the solvent.

[0059] Further preferred examples of solvents include alcohols such as ethanol and isopropanol; amides such as N,N-dimethylformamide, N-methylpyrrolidone, and N,N-dimethylacetamide; ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; and hydrocarbons such as toluene and xylene.

[0060] The oxide dispersion of this disclosure may further contain components other than the oxide (X) and the solvent (also referred to as other components). Examples of other components include binders, antioxidants, dehydrating agents, thickeners, defoaming agents, leveling agents, and other coating materials. The content of these components can be appropriately set within a range that does not impair the effects of the present invention. In the oxide dispersion of this disclosure, the content of other components is, for example, 50 parts by mass or less, 30 parts by mass or less, or 10 parts by mass or less, based on 100 parts by mass of oxide (X) contained in the oxide dispersion.

[0061] The solid content concentration of the oxide dispersion is, for example, 1 to 70% by mass, may be 2 to 65% by mass, or 2 to 50% by mass. Here, "solid content concentration" refers to the proportion of components other than the solvent relative to the total volume of the oxide dispersion.

[0062] <Energy Storage Devices> The energy storage devices of this disclosure (hereinafter also referred to as "the Devices") include a solid electrolyte containing the oxide of this disclosure. Examples of the Devices include secondary batteries and capacitors. When the Devices are secondary batteries, one embodiment is an all-solid-state battery, and sodium-ion secondary batteries are preferred in that they have excellent Na ion conductivity.

[0063] An all-solid-state sodium-ion secondary battery, which is one embodiment of this device, will be described. The sodium-ion secondary battery is a laminate comprising an electrode layer consisting of a positive electrode and a negative electrode, and a solid electrolyte layer, wherein the solid electrolyte layer is arranged between the positive electrode layer and the negative electrode layer so that the solid electrolyte layer and the electrode layer are in contact.

[0064] The materials constituting the positive electrode layer and the negative electrode layer are not particularly limited and can be appropriately selected and used from materials known as electrode materials for sodium-ion secondary batteries. For example, the positive electrode layer may consist of a positive electrode current collector and a positive electrode mixture layer. As the positive electrode current collector, metal foil such as aluminum, titanium, or stainless steel can be used. The positive electrode mixture layer is a layer containing a positive electrode active material and is arranged on the surface of the positive electrode current collector. The positive electrode active material is not particularly limited as long as it is capable of intercalating and releasing sodium. ２ FeP２ O ７ , NaFePO ４ Na ３ V ２ (PO ４ ) ３ NaNi ０．５ Mn ０．５ O ２ Examples of transition metal oxides include the following. The positive electrode mixture layer may also contain, if necessary, solid electrolyte powder, conductive additives (e.g., carbon), binders, etc. Oxide (X) is preferably used as the solid electrolyte powder.

[0065] In the sodium-ion secondary battery of this disclosure, the solid electrolyte layer is formed from a sintered body obtained by sintering an oxide (X). The solid electrolyte layer can be obtained, for example, by molding a material for a solid electrolyte containing an oxide (X) into a desired shape and then sintering it. For molding and sintering methods, refer to the descriptions of molding and sintering methods for the sintered body. The shape of the molded body is not particularly limited and can be appropriately set according to the shape of the energy storage device to which it is applied. The shape of the molded body can be, for example, rectangular or circular. The thickness of the solid electrolyte layer is not particularly limited and can be appropriately set according to the application of the secondary battery, etc. The thickness of the solid electrolyte layer is, for example, 5 to 500 μm. In addition, a solid electrolyte layer of an energy storage device having a desired thickness can be formed by stacking multiple molded bodies.

[0066] The method for manufacturing a sodium-ion secondary battery is not particularly limited, and known methods can be appropriately adopted depending on the battery structure, etc. For example, a laminate comprising a positive electrode layer, a solid electrolyte layer, and a negative electrode layer may be manufactured by sandwiching a solid electrolyte layer formed using an oxide (X) between a positive electrode layer and a negative electrode layer, and preferably by performing a heat treatment and / or pressurization treatment for bonding. Alternatively, an electrolyte material containing granular oxide (X) may be sandwiched between a positive electrode layer and a negative electrode layer and housed in a container, and the container may be subjected to a heat treatment for firing and preferably a pressurization treatment for bonding to manufacture a laminate comprising a positive electrode layer, a solid electrolyte layer, and a negative electrode layer. The laminate comprising a positive electrode layer, a solid electrolyte layer, and a negative electrode layer is usually housed in a case and used as a secondary battery.

[0067] In the case where the device is a capacitor, one embodiment includes a positive electrode layer, a negative electrode layer, and a solid electrolyte layer, wherein the solid electrolyte layer is positioned between the positive electrode layer and the negative electrode layer so that the solid electrolyte layer and the electrode layer are in contact.

[0068] The energy storage device comprising the solid electrolyte of this disclosure can be applied to a variety of applications. Specifically, it can be used as a power source in various mobile devices such as mobile phones, personal computers, smartphones, game consoles, and wearable devices; various mobile devices such as electric vehicles, hybrid vehicles, robots, and drones; and various electrical and electronic devices such as digital cameras, video cameras, music players, power tools, and home appliances.

[0069] The present invention will be described in detail below based on the following examples. However, the present invention is not limited to these examples. In the following, "parts" and "%" mean "parts by mass" and "% by mass," respectively, unless otherwise specified.

[0070] <<Production of Oxides>> (1) Example 1 To obtain the molar ratio shown in Example 1 of Table 1 (Na:Ta:Te:Zr:Si:P = 3.30:0.10:0.05:1.85:2.40:0.60), 1.279 g of sodium carbonate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 0.162 g of tantalum(V) oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 0.058 g of tellurium(IV) oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 1.667 g of zirconium oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 1.054 g of silicon dioxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 0.505 g of ammonium dihydrogen phosphate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were weighed out as the supply components. Each sample was placed in a mortar, 20 g of pure water was added, and wet mixing was performed to obtain the mixture. The resulting mixture was dried at 100°C for 3 hours, then transferred to an alumina crucible (capacity 30 mL), and the temperature was raised to the calcination temperature (1,100°C) over 5.5 hours and held for 5 hours to perform calcination. After that, it was allowed to cool to room temperature to obtain the calcined product. From the obtained calcined product, a φ5 mm ZrO was collected. ２ Using balls, the material was ground in an ethanol dispersion medium for 1 hour using a planetary ball mill. The resulting dispersion was then passed through a 330-mesh sieve and dried at 80°C. 0.3 g of the calcined material was placed in a 1.2 cm diameter mold and molded into a coin shape using a hydraulic press with a load of 1 ton. The resulting molded material was placed on a platinum plate and heated to 800°C over 30 minutes. Then, it was further heated to the main firing temperature (1,300°C, see Table 1) over 2.5 hours and held for 4 hours for the main firing. Afterward, it was allowed to cool to room temperature to obtain the oxide (coin-shaped sintered pellet) of Example 1.

[0071] (2) Examples 2-23 and Comparative Examples 1-3 In the same manner as in Example 1, each raw material was weighed to the molar ratios shown in Examples 2-23 and Comparative Examples 1-3 in Table 1 and placed in a mortar. 20 g of pure water was further added to the mortar and wet mixing was performed to obtain a mixture. The obtained mixture was subjected to calcination and final calcination under the same conditions as in Example 1 to obtain the oxides (coin-shaped sintered pellets) of Examples 2-23 and Comparative Examples 1-3. In Examples 2, 3 and Comparative Example 2, scandium oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for Sc. In Examples 3, 4, 5, and 7, germanium oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for Ge. In Examples 5, 8, 13, 14, 22, and 23, antimony oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for Sb. In Examples 6, 12, 15, and 20, aluminum oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for Al. In Examples 6 and 16, tungsten oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for W. In Examples 7 and 9, yttrium oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for Y. In Examples 8, 12, 17, 18, and 21, bismuth oxide (manufactured by Nippon Chemical Industrial Co., Ltd.) was used as the raw material for Bi. In Examples 10, 15, 23 and Comparative Example 3, indium oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for In. In Example 11 and Comparative Example 3, tin oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for Sn. In Example 13, dysprosium oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for Dy. In Examples 14, 19, and 21, calcium hydroxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for Ca. In Example 16 and Comparative Example 2, magnesium oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for Mg. In Example 17, lanthanum oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for La. In Example 20, gallium oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for Ga. In Example 22, ytterbium oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the raw material for Yb.

[0072] ≪Oxide Characteristics≫ (1) Identification of Elemental Composition (Molar Ratio of Zr / Si) For each oxide in Examples 1 to 23 and Comparative Examples 1 to 3, elemental composition analysis was performed using inductively coupled plasma (ICP) emission spectrometry (ARCOS SOP, Spectro). The analytical samples were prepared as follows: 0.05 g of each oxide was weighed and rinsed in 10 mL of ultrapure water to prepare a slurry. 1 mL of sulfuric acid and 2 mL of hydrofluoric acid were added to this slurry, and the oxide was completely dissolved on a hot plate at 220°C. The dissolved sample was diluted to 50 mL with ultrapure water and used for measurement. The molar ratio of Zr / Si was calculated from the measurement results, and the results are shown in Table 2.

[0073] (2) Measurement of relative density The relative density of each oxide in Examples 1 to 23 and Comparative Examples 1 to 3 was calculated, and the results are shown in Table 2. The method for calculating relative density is as follows: The diameter, thickness, and mass of the oxides produced above were measured, and the measured density was calculated from the measured values ​​of volume and mass. Then, the relative density was calculated by calculating the ratio (%) of the measured density to the theoretical density.

[0074] (3) Identification of the Crystal Phase The main crystal phase of each oxide in Examples 1 to 23 and Comparative Examples 1 to 3 was identified by crystal structure analysis using X-ray diffraction (XRD). The XRD measurement conditions were as follows: X-ray diffraction analyzer: Bruker AXS D8 ADVANCE Characteristic X-ray: CuKα Measurement voltage: 40 kV Measurement current: 40 mA Measurement method: Continuous measurement range: 10° ≤ 2θ ≤ 80° Step side: 0.01° Scan speed: 2.5° / min In all oxides of Examples 1 to 23 and Comparative Examples 1 to 3, the main crystal phase was monoclinic or rhombohedral Na ３ Zr ２ Si ２ PO １２ They were attributed to [a specific group / group], and it was confirmed that all of them possess a NASICON-type crystal structure.

[0075] <<Evaluation of Oxides>> (1) Evaluation of Ionic Conductivity (1-1) Formation of Current Collector Layer Each oxide (coin-shaped sintered pellet) obtained in Examples 1 to 23 and Comparative Examples 1 to 3 was masked with polyimide tape so that a circular exposed surface with a diameter of 6 mm was formed in the center of both sides. Then, a current collector layer was formed on the exposed surface of the sintered pellet by sputtering. The current collector layer was a gold (Au) layer with a thickness of approximately 50 nm. A gold deposition apparatus (Eikoh Co., Ltd., Ion Coater IB-2 / IB-3) was used for sputtering.

[0076] (1-2) AC Impedance Measurement In (1-1) above, the AC impedance of each oxide in Examples 1 to 23 and Comparative Examples 1 to 3, in which the current collector layer was formed, was measured and a complex impedance plot was created. For Examples 1 to 23 and Comparative Examples 1 to 3, measurements were taken using an impedance analyzer (Keysight Corporation, model "E4990A") at frequencies from 20 Hz to 120 MHz and voltages of 10 mV.

[0077] (1-3) Calculation of Ionic Conductivity The value at the right end of the arc in the complex impedance plot obtained in (1-2) above was taken as the resistance R of each oxide (sum of intra-grain and grain boundary resistances), and the ionic conductivity σ (Na ionic conductivity) was calculated using the following formula. The results are shown in Table 2. σ = (t / A) × (1 / R) σ: Ionic conductivity t: Thickness of the sample A: Area of ​​the electrode R: Resistance of the oxide

[0078] (1-4) Calculation of Intragranular Ionic Conductivity In the complex impedance plots of Examples 1 to 23 and Comparative Examples 1 to 3 obtained in (1-2) above, in cases where two arc waveforms were observed, the diameter of the first arc was taken as the intragranular resistance (Rb), and the intragranular ionic conductivity σb (intragranular Na ionic conductivity) was calculated using the following formula. The results are shown in Table 2. σb = (t / A) × (1 / Rb) σb: Ionic conductivity t: Thickness of the sample A: Area of ​​the electrode Rb: Resistance within the grain

[0079] (2) Measurement of curvature The curvature of each oxide in Examples 1 to 23 and Comparative Examples 1 to 3 was calculated, and the results are shown in Table 2. The calculation method is as follows: Curvature Nr = {(D1 - D2) / (D1 + D2)} × 100 D1: Diameter of the convex surface of the sintered pellet (radius of curvature of the convex surface × 2) D2: Diameter of the concave surface of the sintered pellet (radius of curvature of the concave surface × 2) If the curvature Nr is less than 0.5, it is evaluated as ○, if it is 0.5 or more and less than 1.0, it is evaluated as △, and if it is 1.0 or more, it is evaluated as ×.

[0080]

[0081]

[0082] <<Evaluation Results>> As is clear from the results of Examples 1 to 23, oxide (X) showed high Na ion conductivity (especially intragranular Na ion conductivity) and good Na ion conductivity. Furthermore, oxide (X) made it possible to produce sintered bodies with low curvature. From these results, it was found that oxide (X) is useful as a solid electrolyte for Na ions as a carrier for ion conduction.

[0083] In contrast, the oxides without doping elements (Comparative Example 1), those with doping elements consisting only of elements with an electronegativity of 1.70 or less (Comparative Example 2), and those with a Zr / Si ratio of 0.51 (Comparative Example 3) failed to achieve both high Na ion conductivity and low curvature compared to the oxides in Examples 1 to 23.

[0084] From these results, it was found that oxides containing doped elements with an electronegativity of 1.70 or higher, and having a Zr / Si molar ratio of 0.60 to 0.87, exhibit high Na ion conductivity (especially high intragranular Na ion conductivity) and can suppress bending during the fabrication of sintered bodies.

[0085] The present invention is not limited to the embodiments described above, and encompasses various modifications and variations within the scope of equivalents, without departing from the spirit of the invention. Therefore, various combinations and forms, as well as other combinations and forms that include only one, more, or fewer of these elements, should be understood to fall within the scope and conceptual range of the present invention in light of the above teachings.

Claims

1. An oxide containing Na, Zr, Si, and P as constituent elements, wherein the molar ratio of Zr to Si is 0.60 or more and 0.87 or less, and further contains a specific element that is different from Si and P and has an electronegativity of 1.70 or more.

2. The oxide according to claim 1, wherein the electronegativity of the specified element is 2.60 or less.

3. The oxide according to claim 1, satisfying the following general formula (1). (In general formula (1), M1 includes elements that form divalent cations, M2 includes elements that form trivalent cations, M3 includes elements that form tetravalent cations excluding Zr and Si, M4 includes elements that form pentavalent cations excluding P, and M5 includes elements that form hexavalent cations. One or more of M1, M2, M3, M4, and M5 include the specified elements. a, b, c, d, and e are each independently 0 or greater, satisfying "0 < a + b + c + d + e < 2", "2a + b - d - 2e + α > 0", and "0 < α < 1".) 4. The oxide according to claim 1, having a NASICON-type crystal structure.

5. A solid electrolyte containing the oxide described in any one of claims 1 to 4.

6. The solid electrolyte according to claim 5, wherein the relative density is 80% or more.

7. An energy storage device comprising the solid electrolyte described in claim 5.

8. An oxide dispersion containing the oxide described in any one of claims 1 to 4 and a solvent.

9. A method for producing an oxide according to claim 3, comprising the steps of: mixing a supply component containing at least one selected from the group consisting of Na, Zr, Si, P and the specified element in such a way that it satisfies the stoichiometric ratio represented by the general formula (1) above to obtain a mixture of the supply component; and calcining the mixture to obtain the oxide.

10. The method for producing an oxide according to claim 9, wherein the mixing of the supply components is a wet mixing method.

11. A method for producing an oxide according to claim 9 or 10, wherein the mixture is calcined at 800°C or higher.

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