Composition, fired product, solid electrolyte, and power storage device

Abstract

Provided is a composition containing an oxide containing Na, Al, Zr and P as constituent elements, and a sintering aid.

Description

Composition, fired product, solid electrolyte, and electricity storage device

[0001] [Cross-reference to related applications] This application claims priority to Japanese Patent Application No. 2023-211395 filed on December 14, 2023, and Japanese Patent Application No. 2023-219373 filed on December 26, 2023, the entireties of which are incorporated herein by reference. The present disclosure relates to a composition, a fired product, a solid electrolyte, and an electricity storage device.

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

[0003] Lithium-ion secondary batteries have a negative electrode, a positive electrode, and an electrolyte, and are charged and discharged by transferring lithium ions between the electrodes via the electrolyte. Conventionally, nonaqueous electrolytes have been used primarily as the electrolyte. However, because nonaqueous electrolytes contain flammable organic solvents, there are concerns about electrolyte leakage and short circuits within the battery due to overcharge and overdischarge.

[0004] In response to this, all-solid-state lithium-ion secondary batteries that use a solid electrolyte with lithium ion conductivity instead of a non-aqueous electrolyte have been studied. However, concerns have arisen about the rising price of the raw material lithium and its depletion.

[0005] In recent years, various studies have been conducted on sodium ion secondary batteries (NIBs), which use sodium, an abundant and inexpensive resource, as a post-lithium ion secondary battery that uses an element to replace lithium, a rare metal, and are charged and discharged by the movement of sodium ions (see, for example, Patent Document 1).

[0006] Known solid electrolytes with sodium ion conductivity include sulfide-based solid electrolytes and oxide-based solid electrolytes. Of these, oxide-based solid electrolytes are considered to be highly stable under atmospheric conditions and to be excellent in safety. NASICON, a Na-based solid electrolyte, is one of the oxide-based solid electrolytes that exhibits sodium ion conductivity. ３ Zr ２ Si ２ P.O. １２ It is known that NASICON can be doped with various elements to improve its ionic conductivity, and various studies have been conducted (see, for example, Non-Patent Document 1). Non-Patent Document 1 discloses that Na ion conductivity can be increased by doping NASICON with cerium (Ce).

[0007] International Publication No. 2019 / 003846

[0008] ACS APPLIED MATERIALS & INTERFACES, 2020, No. 12, PP. 3502-3509

[0009] To achieve high capacity and high power output in all-solid-state lithium-ion secondary batteries, it is necessary to develop a solid electrolyte that exhibits high sodium ion conductivity. However, oxide-based solid electrolytes are hard materials, which tend to result in poor bonding between particles and at the interfaces between the electrodes and the electrolyte, resulting in high resistance when used as an electrolyte. Therefore, to achieve high ionic conductivity, it is conceivable to improve the bonding between particles and between the electrodes and the electrolyte by firing at high temperatures, for example, above 1,000°C, thereby reducing resistance as much as possible.

[0010] However, when manufacturing an all-solid-state battery, if a firing process at a high temperature exceeding 1,000°C is required to achieve high ionic conductivity, there is a concern that co-sintering of the positive and negative electrodes with the solid electrolyte may not be possible in order to avoid damage to the positive and negative electrodes. On the other hand, if the firing temperature is set to a relatively low temperature of 1,000°C or less, there is a concern that grain boundary resistance and interface resistance may not be sufficiently reduced, resulting in a decrease in Na ion conductivity.

[0011] The present disclosure has been made in view of the above circumstances, and has as its object to provide a composition that can give a fired product having excellent Na ion conductivity by firing at 1,000° C. or less. Another object is to provide a solid electrolyte containing the fired product, and an electricity storage device including the solid electrolyte.

[0012] The present inventors have conducted extensive research to solve the above-mentioned problems and have found that the above-mentioned problems can be solved by a composition containing an oxide containing a specific element and a sintering aid. The present disclosure provides the following composition, sintered product, solid electrolyte, and electricity storage device.

[0013] [1] A composition containing an oxide containing Na, Al, Zr, and P as constituent elements, and a sintering aid. [2] The composition of [1], wherein the oxide further contains Si as a constituent element. [3] The composition of [1] or [2], wherein the sintering aid contains at least one element selected from the group consisting of Na, B, Bi, Zn, Nb, and P as a constituent element. [4] Any of the compositions of [1] to [3], wherein the sintering aid has a melting point of 1,000°C or less. [5] Any of the compositions of [1] to [4], wherein the content of the sintering aid is 0.1 to 45.0 parts by mass per 100 parts by mass of the oxide. [6] Any of the compositions of [1] to [5], wherein two or more types of sintering aids are contained. [7] The composition of any one of [1] to [5], comprising one or more types of the sintering aid, wherein the sintering aid contains Na and B as constituent elements in the same molecule, or Na and B in different molecules. [8] The composition of any one of [1] to [7], which is for a solid electrolyte. [9] A fired product of any one of the compositions of [1] to [8].

[10] The fired product of [9], which has a NASICON-type crystal structure.

[11] The fired product of

[10] , which has a crystal structure represented by the following formula (1): (In formula (1), M1 contains an element that forms a divalent cation, M2 contains an element other than Al that forms a trivalent cation, M3 contains an element (excluding Zr and Si) that forms a tetravalent cation, and a, b, c, d, and e satisfy "a≧0," "b>0," "c≧0," "d≧0," "0≦e<3," and "a+b+c+d<2.")

[12] The fired product of

[11] , which satisfies b≦0.5.

[13] The fired product of

[11] or

[12] , which satisfies 1.5≦e≦2.8.

[14] The fired product of any of [9] to

[13] , which has a relative density of 80% or more.

[15] A solid electrolyte comprising a fired product of any of [9] to

[14] .

[16] An electricity storage device comprising the solid electrolyte of

[15] .

[0014] According to the present disclosure, a fired product exhibiting high Na ion conductivity can be obtained by firing at 1,000° C. or less. Furthermore, by using the fired product as an electrolyte material for an electricity storage device such as a secondary battery or capacitor, it is possible to obtain an electricity storage device that combines high Na ion conductivity with the safety ensured by solidifying the electrolyte.

[0015] FIG. 1 is a diagram showing the results of DC polarization measurement of the sintered body obtained in Example 1. FIG. 2 is a cyclic voltammogram of the sintered body obtained in Example 1. FIG. 3 is a diagram showing the results of critical current density measurement. FIG. 4 is a diagram showing the evaluation results of a charge-discharge cycle test of a half cell using hard carbon (HC). FIG. 5 is a diagram showing the evaluation results of a half cell using Na ３ V ２ (P.O. ４ ) ３ Fig. 6 is a diagram showing the evaluation results of a charge-discharge cycle test of a half cell using (NVP). Fig. 6 is a diagram showing the evaluation results of the charge-discharge rate characteristics of a half cell using HC. Fig. 7 is a diagram showing the evaluation results of the charge-discharge rate characteristics of a half cell using NVP. Fig. 8 is a diagram showing the charge-discharge cycle test results of a full cell.

[0016] The composition, fired product, solid electrolyte, and electricity storage device of the present disclosure are each described in detail below. <Composition> The composition of the present disclosure (hereinafter also referred to as "the composition") contains an oxide containing Na, Al, Zr, and P as constituent elements, and a sintering aid. By firing the composition, a fired product having Na ion conductivity can be obtained. The fired product is useful as a solid electrolyte material, and is particularly useful in that a fired product exhibiting high Na ion conductivity can be obtained even by a low-temperature firing process at 1,000°C or less. Each component contained in the composition is described in detail below. Hereinafter, an oxide containing Na, Al, Zr, and P as constituent elements is also referred to as "oxide (X)."

[0017] <Oxide (X)> The oxide (X) may contain, as constituent elements, Na, Al, Zr, and P. The oxide (X) may have Na ion conductivity in a state where it is obtained by firing a mixture with a sintering aid to form a fired product, and it does not matter whether the oxide (X) has Na ion conductivity in a state before firing the mixture with a sintering aid (i.e., the oxide (X) itself).

[0018] The oxide (X) is NASICON, which is Na ３ Zr ２ Si ２ P.O. １２ Examples of oxides include oxides in which some of the elements constituting NZSP (also referred to as "NZSP") are substituted with various elements including at least aluminum (Al) (in addition to Al, for example, B, Mg, Ca, Ba, Ga, Ge, Sc, Y, Fe, Sr, In, Ti, Hf, Sn, Pb, V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, Ag, Te, Tl, Ta, Sb, Bi, W, and lanthanoid elements). The oxide (X) may have a NASICON structure before the mixture with the sintering aid is fired, or may not have a NASICON structure before the mixture with the sintering aid is fired. The oxide (X) preferably has a composition ratio (molar ratio) determined so that the fired product of the mixture containing the oxide (X) and the sintering aid has a NASICON structure, since this allows for the production of a fired product with high Na ion conductivity.

[0019] In order to obtain a fired product with high Na ion conductivity by firing a mixture of oxide (X) and a sintering aid, oxide (X) is preferably an oxide of Na, Al, Zr, Si, and P in a molar ratio that satisfies the following formula (1): In other words, the oxides in the fired product of the present composition are preferably substances that satisfy the following formula (1): (In formula (1), M1 includes an element that forms a divalent cation, M2 includes an element other than Al that forms a trivalent cation, M3 includes an element (excluding Zr and Si) that forms a tetravalent cation, and a, b, c, d, and e satisfy "a≧0," "b>0," "c≧0," "d≧0," "0≦e<3," and "a+b+c+d<2.")

[0020] The fired material containing a substance satisfying the above formula (1) is Na ３ Zr ２ Si ２ P.O. １２ In an oxide having a basic skeleton of the above, at least a portion of Zr is substituted with Al, and a portion of Zr may be further substituted with at least one of M1 (including an element that becomes a divalent cation), M2 (including an element that becomes a trivalent cation other than Al), and M3 (including an element that becomes a tetravalent cation other than Zr and Si).

[0021] In the above formula (1), examples of M1 include elements that form divalent cations among Group 2 elements, Group 12 elements, and transition elements (Groups 3 to 11 elements), as well as Sn and Pb.

[0022] Examples of M2 include Group 3 elements, Group 13 elements other than Al, transition elements (Groups 3 to 11 elements) that form trivalent cations, Sb, Bi, etc. As the Group 13 element other than Al, B (boron) is preferred.

[0023] M3 is an element other than Zr and Si that forms a tetravalent cation. Examples of M3 include Group 14 elements other than Si, transition elements (Groups 3 to 11 elements) other than Zr that form a tetravalent cation (Ti, Ce), Pb, Se, Te, etc.

[0024] In the above formula (1), a, b, c, d, and e are not particularly limited as long as they satisfy the conditions "a≧0", "b>0", "c≧0", "d≧0", "0≦e<3", and "a+b+c+d<2". For example, when a=0, b>0, c>0, d>0, and e>0, the substance is "Na １＋ｂ＋ｃ＋ｅ Al ｂ M2 ｃ M3 ｄ Zr ２－ｂ－ｃ－ｄ Si ｅ P ３－ｅ O １２ ". When a>0, b>0, c>0, d>0 and e=0, the substance is "Na １＋２ａ＋ｂ＋ｃ M1 ａ Al ｂ M2 ｃ M3 ｄ Zr ２－ａ－ｂ－ｃ－ｄ P ３ O １２ " is expressed as:

[0025] More specifically, a, b, c, d, and e preferably satisfy a≦0.3, more preferably a≦0.2, and even more preferably a≦0.1, in that an impurity phase is unlikely to be formed and a solid electrolyte exhibiting high Na ion conductivity can be obtained. When a>0, the lower limit of a preferably satisfies a≧0.01, and more preferably a≧0.03.

[0026] Regarding b, in that a solid electrolyte exhibiting high Na ion conductivity can be obtained by low-temperature calcination, it is preferable that b≧0.01 be satisfied, more preferably b≧0.02 be satisfied, even more preferably b≧0.03 be satisfied, and even more preferably b≧0.04 be satisfied. Regarding the upper limit of b, it is preferable that b≦0.5 be satisfied, more preferably b≦0.4 be satisfied, even more preferably b≦0.3 be satisfied, even more preferably b≦0.2 be satisfied, and even more preferably b≦0.1 be satisfied.

[0027] Regarding c, in order to suppress the formation of impurity phases and thereby obtain a solid electrolyte having higher Na ion conductivity, it is preferable that c≦0.3, more preferably c≦0.2, and even more preferably c≦0.1. Furthermore, when c>0, the lower limit of c is preferably c≧0.01, and more preferably c≧0.03.

[0028] Regarding d, in order to suppress the formation of impurity phases and obtain a solid electrolyte exhibiting higher Na ion conductivity, it is preferable that d≦0.3 be satisfied, more preferably d≦0.2 be satisfied, and even more preferably d≦0.1 be satisfied. When d>0, the lower limit of d is preferably d≧0.01, and more preferably d≧0.03 be satisfied.

[0029] It is preferable that e satisfies 1.5≦e≦2.8. Regarding the upper limit of e, since it is thought that if the Si substitution amount is too high, the Na ion conductivity will decrease, it is more preferable that e≦2.7, even more preferable that e≦2.6, and even more preferable that e≦2.5. Regarding the lower limit of e, in order to obtain a solid electrolyte exhibiting higher Na ion conductivity, it is more preferable that e≧1.7, even more preferable that e≧1.9, even more preferable that e≧2.1, even more preferable that e≧2.2, and even more preferable that e≧2.3.

[0030] In the above formula (1), the stoichiometric ratio of O is written as 12, but as long as the charge neutrality of the oxides in the fired product as a whole can be maintained, the stoichiometric ratio of O does not have to be strictly 12. In other words, the O derived from the oxides in the fired product may be a value less than 12 or a value greater than 12, as long as the charge neutrality of the oxides in the fired product as a whole can be maintained.

[0031] In order to obtain a sintered product exhibiting higher Na ion conductivity, it is preferable that the oxide (X) further contains silicon (Si) as a constituent element.For example, when the oxide in the sintered product of the present composition containing the oxide (X) and a sintering aid satisfies the above formula (1), e in the above formula (1) preferably satisfies e>0, more preferably satisfies e≧0.25, more preferably satisfies e≧0.5, even more preferably satisfies e≧1.0, and even more preferably satisfies e≧1.5, in order to obtain a sintered product exhibiting higher Na ion conductivity.

[0032] The melting point of the oxide (X) is, for example, 1,050°C or higher, and may be 1,100°C or higher, 1,150°C or higher, 1,200°C or higher, or 1,250°C or higher. There are no particular limitations on the upper limit of the melting point of the oxide (X). The melting point of the oxide (X) is a value under atmospheric pressure.

[0033] There are no particular limitations on the method for producing an oxide (i.e., oxide (X)) for obtaining a fired product containing an oxide that satisfies the above formula (1). The oxide (X) can be produced, for example, by weighing and mixing raw materials so as to satisfy the stoichiometric ratio of the composition represented by the above formula (1) (mixing step), and then firing the resulting mixture. In this specification, firing of raw materials performed to obtain the oxide (X) is referred to as "pre-firing," and this step is referred to as the "pre-firing step."

[0034] As the raw material for oxide (X), a Na supply component, an Al supply component, an M1 supply component, an M2 supply component, an M3 supply component, a Zr supply component, a Si supply component, and a P supply component can be used, whichever corresponds to the element for obtaining the target oxide (X). For example, when producing an oxide for obtaining a sintered body in which a = 0, b > 0, c = 0, d = 0, and e > 0 in the above formula (1), a Na supply component, an Al supply component, a Zr supply component, a Si supply component, and a P supply component are used as the raw material for oxide (X). Note that these supply components may be compounds in which one type of supply component contains two or more elements selected from Na, Al, M1, M2, M3, Zr, Si, and P.

[0035] The raw materials for oxide (X), i.e., the Na supply component, Al supply component, M1 supply component, M2 supply component, M3 supply component, Zr supply component, and Si supply component, may be inorganic compounds or organic compounds. For each supply component, for example, carbonate, hydrogencarbonate, sulfate, sulfite, nitrate, nitrite, phosphate, acetate, citrate, ammonium salt, oxide, hydroxide, chloride, sulfide, etc. of these metal elements can be used. Phosphates are preferably used as the P supply component, and sodium phosphate or ammonium dihydrogen phosphate is particularly preferred.

[0036] In producing the oxide (X), the raw materials may be mixed in the mixing step by dry mixing or by wet mixing using a liquid. Of these, wet mixing is preferred. 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 be subjected to removal of coarse particles or aggregates using, for example, a sieve.

[0037] Next, the mixture obtained in the mixing step is pre-baked (pre-baking step). The baking temperature in the pre-baking step is not particularly limited, but can be, for example, 900°C or higher, preferably 950°C or higher, and more preferably 1,000°C or higher. The upper limit of the baking 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 pre-baking, the temperature may be increased stepwise from a temperature lower than the baking temperature, and finally maintained at the temperature required for baking.

[0038] The oxide (X) obtained by the calcination step may be pulverized by any method, for example, using a pulverizer such as a ball mill, a planetary ball mill, a bead mill, a jet mill, or a blender.

[0039] <Sintering Aid> The sintering aid is blended into the composition as a component separate from the oxide (X) for purposes such as promoting sintering of the oxide (X). In particular, the sintering aid is used in the composition to impart excellent Na ion conductivity to the fired product obtained by firing the oxide (X). From the viewpoint of imparting excellent Na ion conductivity to the fired product obtained by firing the composition, a compound containing at least one element selected from the group consisting of Na, B, Bi, Zn, Nb, and P (hereinafter also referred to as "specific element") as a constituent element can be preferably used as the sintering aid. In view of the ease of raw material availability, a compound containing one or two elements selected from Na, B, Bi, Zn, Nb, and P is preferred as the sintering aid.

[0040] Specific examples of the sintering aid containing a specific element include oxides and carbonates containing the specific element. Further specific examples of the sintering aid 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 P.O. ４ , NaH ２ P.O. ４ ・2H ２ O, Na ３ P.O. ４ , Na ３ P.O. ４ ・12H ２ O, 60Na ２ O-10Nb ２ O ５ -30P ２ O ５ etc.

[0041] The melting point of the sintering aid is preferably lower than the melting point of the oxide (X). From the viewpoint of obtaining a fired product exhibiting high Na ion conductivity by firing at as low a temperature as possible, the melting point of the sintering aid is preferably 1,000°C or lower, more preferably 950°C or lower, and even more preferably 900°C or lower. The lower limit of the melting point of the sintering aid is not particularly limited, but may be, for example, 350°C or higher, or 400°C or higher. The melting point of the sintering aid is a value under atmospheric pressure.

[0042] The present composition may contain only one type of sintering aid, or may contain two or more types. The present composition preferably contains two or more types of sintering aids, since this allows for the production of a fired product with higher Na ion conductivity. When the present composition contains two or more types of sintering aids, it is preferable that the melting points of at least some of the sintering aids contained in the present composition are lower than the melting point of the oxide (X), and it is more preferable that the melting points of all of the sintering aids are lower than the melting point of the oxide (X).

[0043] Of the above, the sintering aid incorporated into the present composition preferably contains Na (sodium) and B (boron) as constituent elements in the same molecule, or Na and B in different molecules. Incorporating Na and B into the present composition via the sintering aid is advantageous in that the Na ion conductivity of the sintered product obtained by firing the present composition at 1,000°C or less can be increased. From the viewpoint of obtaining a sintered product exhibiting higher Na ion conductivity, the sintering aid preferably contains Na and B in different molecules. Specifically, the present composition preferably contains a first compound and a second compound different from the first compound as sintering aids, the first compound preferably containing Na and the second compound preferably containing B. From the viewpoint of easy availability of raw materials, a carbonate can be preferably used as the first compound, and an oxide can be preferably used as the second compound.

[0044] The ratio of Na to B in the sintering aid blended into this composition, expressed as the molar amount of Na per mole of B in the sintering aid (total amount when two or more types are included), is preferably in the range of 0.1 to 10 moles. The molar amount of Na per mole of B in the sintering aid is more preferably 0.2 to 8.0 moles, even more preferably 0.3 to 6.0 moles, even more preferably 0.4 to 5.0 moles, even more preferably 0.45 to 4.0 moles, and even more preferably 0.5 to 3.0 moles. By keeping the ratio of Na to B in the sintering aid within the above range, the Na ion conductivity of the fired product can be further increased.

[0045] The content of the sintering aid in the composition (the total amount when two or more sintering aids are contained) is preferably 0.1 to 45.0 parts by mass per 100 parts by mass of oxide (X). By setting the content of the sintering aid within the above range, a solid electrolyte exhibiting high Na ion conductivity can be obtained. From the viewpoint of sufficiently obtaining the effect of improving Na ion conductivity in a fired product of the composition by incorporating the sintering aid, the content of the sintering aid is preferably 1.0 part by mass or more, more preferably 3.0 parts by mass or more, and even more preferably 5.0 parts by mass or more, per 100 parts by mass of oxide (X). Furthermore, the upper limit of the content of the sintering aid is preferably 40.0 parts by mass or less, more preferably 35.0 parts by mass or less, even more preferably 30.0 parts by mass or less, and even more preferably 25.0 parts by mass or less, per 100 parts by mass of oxide (X), from the viewpoint of suppressing performance degradation caused by incorporating an excessive amount of sintering aid.

[0046] <Other Components> The present composition may be a composition comprising an oxide (X) and a sintering aid, or may further contain a component (other component) different from the oxide (X) and the sintering aid. Examples of other components include a polymer solid electrolyte, a polymer gel electrolyte, an inorganic filler, a conductive additive, a plasticizer, and a binder. However, considering the Na ion conductivity and ease of manufacture of the fired product obtained by firing the present composition, it is preferable that the content of other components be as low as possible. Specifically, the content of other components is preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.1% by mass or less, based on the total amount of the present composition.

[0047] The present composition can be obtained by mixing the oxide (X) with a sintering aid. The oxide (X) and the sintering aid may be mixed, for example, by pulverizing the oxide (X) into a powder and then mixing it with the sintering aid, or by pulverizing the sintering aid by any method and then mixing it with the oxide (X). Alternatively, the oxide (X) and the sintering aid may be mixed and then pulverized into a powder, or the oxide (X) and the sintering aid may be mixed and pulverized simultaneously. The oxide (X) and the sintering aid may be mixed by a method that can homogenize the oxide (X) and the sintering aid. The oxide (X) and the sintering aid may be mixed using various devices, such as a ball mill, a planetary ball mill, a bead mill, a blender, a homogenizer, a stamp mill, a homomixer, or a disperser mixer. Furthermore, when producing a fired product on a small scale, mixing may be performed using a mortar and pestle.

[0048] The oxide (X) and the sintering aid may be mixed by dry mixing or by wet mixing using a liquid. Of these, wet mixing is preferred. By using wet mixing, the dispersibility of the sintering aid can be further improved, and the density of the fired product can be increased compared to dry mixing. This also allows the Na ion conductivity of the fired product to be relatively improved. Water, various organic solvents, and mixtures thereof can be used as the liquid used for wet mixing. The slurry obtained by wet mixing may be subjected to removal of coarse particles or aggregates using, for example, a sieve. Furthermore, the mixing of the oxide (X) and the sintering aid may be performed at room temperature, at a low temperature, or under heating.

[0049] <Method for Producing the Fired Product> The fired product of the present disclosure can be produced by firing the present composition described above. Note that, in this specification, firing the present composition to obtain a fired product that can become a solid electrolyte is referred to as "main firing," and this process is referred to as "main firing process."

[0050] In the firing step, the composition may be fired without molding, or may be fired after molding. The molding method is not particularly limited, and known methods such as extrusion molding, injection molding, pressure molding, slip casting, mold casting, and tape casting can be used. Among these, it is preferable to obtain a molded product by pressure molding, since this minimizes the porosity of the resulting fired product and makes it easier to form ion conduction paths.

[0051] The firing temperature when firing the present composition (main firing) is preferably 1,200°C or lower, more preferably 1,100°C or lower, even more preferably 1,000°C or lower, even more preferably 950°C or lower, even more preferably 900°C or lower, even more preferably 850°C or lower, and particularly preferably 800°C or lower. According to the composition of the present disclosure, a fired product exhibiting high Na ion conductivity can be obtained even when fired at a relatively low temperature of 1,000°C or lower. This enables co-firing with positive electrode materials and negative electrode materials in the manufacturing process of an electric storage device. The lower limit of the firing temperature is preferably 500°C or higher, more preferably 550°C or higher, even more preferably 600°C or higher, even more preferably 650°C or higher, even more preferably 700°C or higher, and even more preferably 750°C or higher, in order to sufficiently reduce the interfacial resistance in the fired product and increase Na ion conductivity. During the main firing, the temperature may be increased in stages from a temperature lower than the firing temperature (pre-firing), and the temperature required for the final firing may be maintained. Furthermore, for the purpose of removing distortion, etc., the fired product may be subjected to a treatment after the main firing in which it is heated at a temperature lower than the firing temperature during the main firing.

[0052] When pre-firing is performed, the resulting pre-firing product may be pulverized before the next firing process. When firing is performed at multiple temperature ranges, firing may be performed in three stages, for example, by performing a first pre-firing in a temperature range of 300°C or higher but lower than 500°C, a second pre-firing in a temperature range of 500°C or higher but lower than 700°C, and a final firing in a temperature range of 700°C or higher. The firing time is not particularly limited, but may be, for example, a pre-firing period of 1 hour or longer but 100 hours or shorter, and a final firing period of 1 hour or longer but 100 hours or shorter. The firing time of the present composition is, for example, 3 to 72 hours, or may be 5 to 48 hours.

[0053] The sintered product thus obtained exhibits excellent ionic conductivity (more specifically, Na ion conductivity). Therefore, the sintered product of the present disclosure is suitable as a solid electrolyte for an electricity storage device in which the ionic carrier is a sodium ion. Furthermore, the present composition is suitable as a composition for a solid electrolyte.

[0054] Specifically, the ionic conductivity measured at 25°C using an AC impedance method for a fired product having a thickness of about 1,000 µm (specifically, 1,000 ± 20 µm) is, for example, 0.1 mS / cm or more. From the viewpoint of obtaining an electricity storage device with excellent performance, the ionic conductivity under the same conditions is preferably 0.5 mS / cm or more, and more preferably 1.0 mS / cm or more. Details of the method for measuring ionic conductivity follow the method described in the Examples below.

[0055] The phase structure of the fired product of the present composition is not particularly limited. The fired product of the present composition preferably has a NASICON-type crystal structure, as this allows for the production of a solid electrolyte with high Na ion conductivity. More specifically, the fired product of the present composition preferably contains an oxide having a NASICON-type crystal structure. Unlike layered structures, the NASICON-type crystal structure provides three-dimensional mobility for alkali metal ions, and zirconium (Zr) is stable even under high voltages, making it useful as a solid electrolyte with high operating voltages. Furthermore, when the fired product of the present composition has the crystal structure represented by formula (1) above, it exhibits excellent Na ion conductivity and is therefore particularly suitable as an electrolyte material. The inclusion of Al as a constituent element in the oxide (X) and the fired product of the present composition is believed to be related to the fact that the resulting fired product maintained high Na ion conductivity even when fired at temperatures below 1,000°C. The crystal structure of the fired product can be determined from the diffraction profile obtained by powder X-ray diffraction measurement.

[0056] The relative density of the fired product of this composition is preferably 75% or more, more preferably 80% or more, even more preferably 85% or more, even more preferably 90% or more, and even more preferably 95% or more, since this allows for a fired product exhibiting excellent Na ion conductivity. In this specification, "relative density" refers to the ratio (%) of the measured density to the theoretical density. The measured density is calculated by measuring the diameter, thickness, and mass of the fired product, and then using the volume calculated from the measured diameter and thickness values ​​and the measured mass value.

[0057] <Electricity Storage Device> The electricity storage device of the present disclosure (hereinafter also referred to as "the device") includes a solid electrolyte containing the fired product of the present disclosure. Specific embodiments of the device include secondary batteries, capacitors, and the like. When the device is a secondary battery, one embodiment is an all-solid-state battery, and a sodium-ion secondary battery is preferred because of its excellent Na-ion conductivity.

[0058] An all-solid-state sodium-ion secondary battery, which is one embodiment of this device, will now be described. A sodium-ion secondary battery is a laminate comprising electrodes consisting of a positive electrode and a negative electrode, and a solid electrolyte layer, with the solid electrolyte layer being disposed between the positive electrode and the negative electrode so that the solid electrolyte layer is in contact with the electrode. There are no particular limitations on the materials constituting the positive electrode and the negative electrode, and any suitable material may be selected from materials known as electrode materials for sodium-ion secondary batteries.

[0059] In the sodium ion secondary battery of the present disclosure, the solid electrolyte layer is formed from a fired product obtained by firing the present composition. The solid electrolyte layer can be obtained, for example, by molding a solid electrolyte material containing the present composition into a desired shape and firing it. The molding method and firing method described above for the fired product can be used for the molding method and firing method. The shape of the molded product is not particularly limited and can be appropriately set depending on the shape of the applied power storage device. The shape of the molded product is, for example, rectangular or circular. The thickness of the solid electrolyte layer is not particularly limited and can be appropriately set depending on the application of the secondary battery, etc. The thickness of the solid electrolyte layer is, for example, 5 to 500 μm. Note that a solid electrolyte layer of an electricity storage device having a desired thickness may be formed by stacking multiple molded products.

[0060] The method for producing 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, a solid electrolyte layer, and a negative electrode may be produced by sandwiching a solid electrolyte layer formed using the present composition between a positive electrode and a negative electrode, preferably by heat treatment and / or pressure treatment for bonding. Alternatively, a laminate comprising a positive electrode, a solid electrolyte layer, and a negative electrode may be produced by sandwiching an electrolyte material containing the present composition in a granular form between a positive electrode and a negative electrode and placing the resulting container therein, and then subjecting the container to heat treatment for sintering and preferably pressure treatment for bonding. The present composition is particularly suitable in that it can produce a solid electrolyte layer exhibiting high Na ion conductivity even through a relatively low-temperature sintering process below 1,000°C. Therefore, it is advantageous in that it allows co-sintering of the electrode and the solid electrolyte layer while minimizing damage to the positive electrode material and the negative electrode material. A laminate comprising a positive electrode, a solid electrolyte layer, and a negative electrode is typically housed in a case and used as a secondary battery.

[0061] The device may be a capacitor. One embodiment of the capacitor includes a positive electrode, a negative electrode, and a solid electrolyte layer, with the solid electrolyte layer disposed between the positive electrode and the negative electrode so that the solid electrolyte layer is in contact with the electrode.

[0062] The power storage device including the solid electrolyte of the present disclosure can be used for various purposes, specifically, as a power source for various mobile devices such as mobile phones, personal computers, smartphones, game consoles, and wearable devices; various moving objects such as electric vehicles, hybrid vehicles, robots, and drones; and various electric and electronic devices such as digital cameras, video cameras, music players, power tools, and home appliances.

[0063] The present disclosure will be specifically described below based on examples. However, the present disclosure is not limited to these examples. In the following, "parts" and "%" mean "parts by mass" and "% by mass", respectively, unless otherwise specified.

[0064] <<Production of Oxides>> [Production Example 1] 4.0607 g of sodium phosphate (manufactured by Kishida Chemical Co., Ltd., purity 98%), 2.8989 g of sodium carbonate (manufactured by Kojundo Chemical Laboratory, purity 99%), 0.0958 g of aluminum oxide (manufactured by Kojundo Chemical Laboratory, purity 99.9%), 9.1565 g of zirconium oxide (manufactured by Kojundo Chemical Laboratory, purity 98%), and 5.2781 g of silicon dioxide (manufactured by Kojundo Chemical Laboratory, purity 99.99%) were weighed out as feed components so as to achieve the molar ratio of oxides shown in Table 1 (Na:Al:Zr:Si:P=3.40:0.05:1.95:2.35:0.65), and each sample was placed in a 500 mL nylon pot containing 200 g of 4 mm diameter zirconia balls, followed by the addition of 65 g of 99.5% ethanol. Using a planetary ball mill (FRITSCH6, Pulerisette6), mixing was carried out for 120 minutes at 200 rpm to obtain a slurry. The obtained slurry was passed through a 250 mesh sieve, dried at 80°C for 12 hours, and then granulated using a 50 mesh sieve. The mixture was then transferred to an alumina crucible (volume 100 mL), heated to 1,100°C in the atmosphere, and maintained for 12 hours to perform pre-firing. At this time, the heating rate was 300°C / hour. The mixture was then allowed to cool to room temperature to obtain oxide SE-1.

[0065] [Production Examples 2 to 4] The same operations as in Production Example 1 were carried out except that the amounts of raw materials charged were changed as shown in Table 1, to obtain oxides (oxide SE-2, oxide SE-3, and oxide SE-4, respectively).

[0066]

[0067] <<Production of Composition and Sintered Product>> [Example 1] The mass ratio of oxide to sintering aid was the mass ratio shown in Table 2 (SE-1:Na ２ CO ３ : B ２ O ３3.6 g of oxide SE-1, 0.301 g of sodium carbonate (manufactured by Kojundo Chemical Laboratory), and 0.099 g of boron trioxide (manufactured by Kishida Chemical Co., Ltd.) were weighed so that the ratio was 100:8.4:2.7), and each sample was placed in a 500 mL nylon pot containing 200 g of φ4 mm zirconia balls, and 65 g of 99.5% ethanol was added. Using a planetary ball mill, mixing was carried out for 18 hours at 250 rpm to obtain a slurry. The obtained slurry was passed through a 250 mesh sieve, dried at 80 ° C. for 12 hours, and then granulated using a 50 mesh sieve to obtain a composition containing oxide SE-1 and a sintering aid. 0.36 g of the obtained composition was placed in a mold with a diameter of 15 cm, and provisionally molded into a cylindrical shape using a hydraulic press under a load of 40 MPa. The mixture was then vacuum-sealed in a plastic bag using a vacuum sealer (TOSEI Corporation, V-280A), placed in a CIP apparatus (Riken Seiki Co., Ltd., P-1B), and molded under a pressure of 200 MPa in water. The obtained molded body was coated with platinum foil, and placed in a 100 mL alumina crucible with the oxide SE-1 so that the platinum foil-coated molded body was embedded in 10 g of oxide SE-1. The main firing was carried out under firing conditions of 900 ° C. and 12 hours. Thereafter, the mixture was allowed to cool to room temperature, and the platinum foil was removed to obtain a fired product of the composition. The heating rate was 300 ° C. / hour.

[0068] [Examples 2 to 14, 16 to 23] Compositions containing oxides and sintering aids were obtained by the same procedure as in Example 1, except that the types and amounts of raw materials were changed as shown in Table 2. Furthermore, using the obtained compositions, molding and main firing were carried out in the same manner as in Example 1 above, to obtain fired products of each composition. Note that in the main firing, the type of oxide in which the platinum foil-coated molded body was embedded was changed to the oxide used in preparing the composition, and the firing conditions were as shown in Table 2.

[0069] [Example 15] As raw materials for the sintering aid, 12.0800 g of sodium carbonate (manufactured by Kojundo Chemical Laboratory Co., Ltd.), 5.0037 g of niobium (V) oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 13.2242 g of ammonium dihydrogen phosphate (manufactured by Kishida Chemical Co., Ltd.) were weighed into a 100 mL alumina crucible, and each sample was placed in a platinum crucible and melted at 1,300°C. Thereafter, the sintering aid (60Na ２ O-10Nb ２ O ５ -30P ２ O ５ ) was obtained. The obtained sintering aid was placed in a 500 mL nylon pot containing 200 g of φ4 mm zirconia balls, and 70 g of 99.5% ethanol was added. The mixture was pulverized at 300 rpm for 12 hours and used to produce a composition. The melting point of the obtained sintering aid was 730°C. Then, each sample was weighed so that the mass ratio of the oxide to the sintering aid was the mass ratio shown in Table 2, and the same operation as in Example 1 above was performed to obtain a composition. The obtained composition was then molded and fired using the same procedure as in Example 1 above to obtain a fired product of the composition.

[0070] [Example 24] The mass ratio of oxide to sintering aid was the mass ratio shown in Table 2 (SE-1:Na ２ CO ３ : B ２ O ３ 3.6 g of oxide SE-1, 0.301 g of sodium carbonate, and 0.099 g of diboron trioxide were weighed out so that the ratio of the oxide SE-1 to the total weight of the powder was 100:8.4:2.7, and the mixture was placed in an agate mortar and mixed for 10 minutes to obtain a composition. The obtained composition was then molded and fired in the same manner as in Example 1 above, to obtain a fired product of each composition.

[0071] Comparative Example 1 Molding and firing were carried out in the same manner as in Example 1, except that no sintering aid was used, to obtain a fired product of the composition.

[0072]

[0073] The details of the compounds used in Table 2 are shown below: ２ CO ３Sodium carbonate, melting point 851°C (manufactured by Kojundo Chemical Laboratory) B ２ O ３ Boron trioxide, melting point 450°C (Kishida Chemical Co., Ltd.) ２ B ４ O ７ Sodium tetraborate, melting point 742°C (Kishida Chemical Co., Ltd.) Bi ２ O ３ : Bismuth (III) oxide, melting point 817°C (Kishida Chemical Co., Ltd.) ZnO: Zinc oxide, melting point 1,975°C (Kanto Chemical Co., Ltd.) 60Na ２ O-10Nb ２ O ５ -30P ２ O ５ : Sintering aid obtained in Example 15, melting point 730°C

[0074] <<Method for Evaluating the Fired Products>> (1) Measurement of Relative Density The relative density of the fired products of each composition of Examples 1 to 24 and Comparative Example 1 was calculated, and the results are shown in Table 2. The calculation method is as follows. The diameter, thickness, and mass of the fired product of each composition 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. The theoretical density is 3.27 g / cm ３ It was decided.

[0075] (2) Identification of Crystalline Phase The main crystalline phase of the fired products of the compositions of Examples 1 to 24 and Comparative Example 1 was identified using X-ray diffraction (XRD) measurement. The XRD measurement conditions were as follows: X-ray diffraction measurement apparatus: D8ADVANCE (manufactured by Bruker) Characteristic X-ray: CuKα Measurement voltage: 40 kV Measurement current: 40 mA Measurement method: Continuous Measurement range: 10°≦2θ≦80° Step side: 0.027° Scan speed: 1.7° / min In the fired products of all the compositions of Examples 1 to 24 and Comparative Example 1, it was confirmed that the main crystalline phase was a monoclinic or rhombohedral NASICON structure.

[0076] (3) Evaluation of Ion Conductivity (3-1) Formation of Current Collector Layer The center of each of the fired compositions obtained in Examples 1 to 24 and Comparative Example 1 was masked with silicone rubber to create a circular exposed surface with a diameter of 7 mm, and sputtering was then performed on the exposed surface. The fired product was then turned over and similarly masked and sputtered to form current collector layers on both sides. The current collector layer was formed as a gold (Au) layer with a thickness of approximately 25 nm. A gold vapor deposition apparatus (V-Tech, VTC-YAN350) was used for sputtering.

[0077] (3-2) Method for Measuring AC Impedance The AC impedance of the fired product on which the current collector layer was formed in (3-1) above was measured, and a complex impedance plot was created. The measurement was performed using a multipotentio / galvanostat (SP-150e, manufactured by Biologic) at a frequency of 100 mHz to 1 MHz, a voltage of 20 mV, and a temperature of 25°C.

[0078] (3-3) Method for calculating ionic conductivity The value at the right end of the arc in the complex impedance plot obtained in (3-2) above was taken as the resistance R of each fired product, and the ionic conductivity σ (Na ion conductivity) was calculated using the following formula (1), and is shown in Table 2. σ = (t / A) × (1 / R) (1) σ: ionic conductivity t: thickness of sample A: area of ​​current collector layer R: resistance of fired product

[0079] (3-4) DC Polarization Measurement The DC polarization measurement was carried out on the fired product of Example 1 on which the current collector layer was formed in the above (3-1). Measurement was carried out using a multipotentio / galvanostat (SP-150e, manufactured by Biologic) at a voltage of 1.0 V and a temperature of 25°C. The results of the DC polarization measurement are shown in Figure 1. The steady-state current was read and the electronic conductivity was calculated according to the following formula (2), which was 2.0 x 10 －８ This confirmed that the fired product of Example 1 had a sufficiently low electronic conductivity. ｅ =(t / A)×(I / V)…(2) σ ｅ : Electronic conductivity t: Thickness of sample A: Area of ​​current collector layer I: Steady-state current V: Voltage

[0080] (3-5) Cyclic Voltammetry Measurement The center of the fired product of Example 1 was masked with silicone rubber to form a circular exposed surface with a diameter of 7 mm, and sputtering was performed on the exposed surface to form a current collector layer on only one side. The current collector layer was formed as a gold (Au) layer with a thickness of approximately 25 nm. A gold vapor deposition device was used for sputtering. After forming the current collector layer on one side of the fired product, it was transferred into a glove box under an argon atmosphere. Metallic Na was placed on the side on which the current collector layer was not formed, and a pressure of 0.2 MPa was applied to cause compression bonding. The fired product with metallic Na attached was sealed in an electrochemical measurement cell (TYS-00DM01, manufactured by EC Frontier Co., Ltd.), and connected to a multipotentio / galvanostat (SP-150e, manufactured by Biologic Co., Ltd.) so that the gold current collector layer side served as the working electrode and the metallic Na side served as the reference electrode and counter electrode. CV measurement was performed at a temperature of 25°C to obtain a cyclic voltammogram. The scan rate was 1 mV / s, starting from the open circuit voltage, and the turnaround voltages were set to -0.1 V and 5.0 V, followed by scanning up to the open circuit voltage. The obtained cyclic voltammogram is shown in Figure 2. 0 to 5.0 V (vs Na + It was confirmed that the fired product was stable in the above-mentioned mixture (Na).

[0081] (3-6) Critical Current Density Measurement In a glove box under an argon atmosphere, metallic Na was placed on both sides of the fired product (thickness: 0.13 cm) of the composition of Example 1, and a pressure of 10 MPa was applied to compress the fired product to a thickness of 0.40 cm. ２ The fired product on which a metal Na electrode with an area of ​​1000 nm was formed was sealed in an electrochemical measurement cell (TYS-00DM01, manufactured by EC Frontier Co., Ltd.) and connected to a multipotentio / galvanostat (SP-150e, manufactured by Biologic Co., Ltd.) so that one side served as the working electrode and the other side as the reference electrode and counter electrode. A current was applied while switching the direction of current flow every 30 minutes (switching from the working electrode to the counter electrode, and from the counter electrode to the working electrode), and the voltage at that time was measured. The measurement was carried out at a temperature of 25°C, with the current value increasing stepwise from 0.04 mA to 0.4 mA for each cycle. The measurement results are shown in Figure 3. -As shown, stable voltage behavior was observed up to a current value of 0.32 mA, and no clear short-circuit behavior was observed up to 0.34 mA. The critical current density was 0.8 mA / cm. ２ From the above, it was confirmed that the fired product of the composition of Example 1 had sufficient short-circuit resistance.

[0082] <Evaluation Results> As is clear from Table 2, the compositions of Examples 1 to 24 were able to give fired products with excellent sodium ion conductivity even when the firing temperature was 1,000° C. or less.

[0083] When the results of Examples 1 to 24 were examined in detail, it was found that when the amount of sintering aid per 100 parts by mass of oxide was changed (Examples 1 to 4), the sodium ion conductivity became more excellent as the amount of sintering aid approached 11.1 parts by mass. ２ CO ３ and B ２ O ３ When these were used together, the Na ion conductivity was highest when the molar ratio of Na to B in the sintering aid was 1:1 during firing at 900°C (Examples 1, 5 to 8).Furthermore, the Na ion conductivity was highest when the molar ratio of Na to B in the sintering aid was 3:2 during firing at 800°C (Examples 18 to 22).

[0084] When the type of oxide was changed (Examples 1, 9 to 11), the Na ion conductivity was best when oxide SE-1 (Na:Al:Zr:Si:P = 3.40:0.05:1.95:2.35:0.65) was used.

[0085] When the type of sintering aid was changed (Examples 1, 12 to 15), Na ２ CO ３ and B ２ O ３ Furthermore, when Example 7 and Example 12 are compared, even if the molar ratio of Na to B in the sintering aid is the same, the Na ion conductivity is the best when Na is used. ２ CO ３ and B ２ O ３ The results showed that the use of sintering aids such as SiO2 and SiO2 resulted in superior Na ion conductivity.

[0086] When the firing temperature of the composition was changed (Examples 6 and 17), the higher the firing temperature, the better the Na ion conductivity. When the firing time was changed (Examples 21 and 23), the longer the firing time, the better the Na ion conductivity.

[0087] Comparing Example 1 and Example 24, in which the composition mixing method was changed, the relative density of the fired product was higher and the Na ion conductivity was superior in Example 1, in which the composition was prepared by mixing using a planetary ball mill, than in Example 24, in which the composition was prepared by mixing using a mortar.

[0088] In contrast, the fired product of Comparative Example 1, to which no sintering aid was added, had a lower relative density and Na ion conductivity than Examples 1 to 24, resulting in poor practicality.

[0089] From the above results, it is clear that a composition containing an oxide (M) and a sintering aid can provide a solid electrolyte that exhibits high relative density and Na ion conductivity even when fired at 1,000°C or less.

[0090] <Battery Evaluation> Battery evaluation was carried out by the following method to confirm that the all-solid-state batteries produced using the compositions of the examples functioned as sodium ion secondary batteries.

[0091] <Method for Producing Half Cells> (1) Method for Producing Half Cells Using Hard Carbon (HC) 0.0605 g of the composition prepared in Example 1 above and 0.0472 g of HC were placed in a 5 mL screw cap bottle, and 0.0204 g of polyvinyl butyral (PVB) and 2.4 g of 99.5% ethanol were added. The mixture was dispersed using an ultrasonic cleaner (manufactured by SND Corporation, US103) to obtain a composite dispersion. Using a spin coater (manufactured by Kyowa Riken Co., Ltd., K359S1), 0.00025 g of the composite dispersion was dropped onto one side of the fired product of the composition prepared in Example 1 above, to form a negative electrode composite precursor layer on the fired product. Thereafter, the negative electrode composite precursor layer was placed facing up on a platinum plate and co-sintered by firing at 950 ° C. for 3 hours in an Ar atmosphere containing 5% hydrogen, forming a negative electrode composite layer on the fired product. Thereafter, the sample was transferred into a glove box under an argon atmosphere, and metallic Na was placed on the surface on which the current collector layer was not formed. A pressure of 0.2 MPa was applied to the sample, and the sample was then compressed and sealed in an electrochemical measurement cell (SP-150e, manufactured by Biologic).

[0092] (2) Na ３ V ２ (P.O. ４ ) ３Method for producing a half cell using NVP (NVP) 20.5315 g of disodium hydrogen phosphate (manufactured by Kishida Chemical Co., Ltd.) and 7.9813 g of vanadium(III) oxide (manufactured by Kishida Chemical Co., Ltd.) were weighed as feed components so that the composition ratio of NVP was (Na:V:P = 3.0:2.0:3.0). Each sample was placed in a platinum crucible, melted at 1,300 ° C, and then poured onto a copper plate to produce an NVP glass precursor. The prepared precursor was placed in a 500 mL nylon pot containing 200 g of φ4 mm zirconia balls, and 70 g of 99.5% ethanol was added. The mixture was pulverized at 300 rpm for 12 hours to obtain a pulverized product. 0.1 g of the pulverized NVP glass precursor and 2.4 g of 99.5% ethanol were placed in a 6 mL screw cap bottle and dispersed to obtain an NVP precursor dispersion. Using a spin coater, 0.00014 g of NVP dispersion was dropped onto one side of the fired product of the composition prepared in Example 1 above, forming an NVP precursor layer on the fired product. The NVP precursor layer was then placed face up on a platinum plate and co-sintered by firing at 750°C for 0.5 hours in an Ar atmosphere containing 5% hydrogen, forming an NVP positive electrode layer on the fired product and obtaining a sintered body. The NVP positive electrode layer of the resulting sintered body was masked with silicone rubber to form a circular exposed surface with a diameter of 7 mm. The exposed surface of the NVP positive electrode layer was then sputtered to form a current collector layer on only one side. The current collector layer was a gold (Au) layer approximately 25 nm thick. A gold vapor deposition device was used for sputtering. The product was then transferred to a glove box under an argon atmosphere. Metallic Na was placed on the side not forming the current collector layer, and a pressure of 0.2 MPa was applied to compress the surface, which was then sealed in an electrochemical measurement cell.

[0093] <Method of Producing a Full Cell> (1) Method of Producing a Full Cell Using NVP and HC 0.0605 g of the composition prepared in Example 1 above and 0.0472 g of HC were placed in a 5 mL screw cap bottle, 0.0204 g of PVB and 2.4 g of 99.5% ethanol were added, and the mixture was dispersed using an ultrasonic cleaner (manufactured by SND Corporation, US103) to obtain a composite dispersion. Using a spin coater (manufactured by Kyowa Riken Co., Ltd., K359S1), 0.00025 g of the composite dispersion was dropped onto one side of the fired product of the composition prepared in Example 1 above, to form a negative electrode composite precursor layer on the fired product. Thereafter, the negative electrode composite precursor layer was placed facing up on a platinum plate and co-sintered by firing at 950 ° C. for 3 hours in an Ar atmosphere containing 5% hydrogen, to form a negative electrode composite layer on the fired product. Then, using a spin coater, 0.00014 g of an NVP glass precursor dispersion prepared using the same procedure as in (2) of the <Half-Cell Preparation Method> was dropped onto the surface opposite the surface on which the negative electrode composite layer was formed, thereby forming an NVP precursor layer on the fired product. The NVP precursor layer was then placed facing up on a platinum plate and co-sintered by firing at 750°C for 0.5 hours in an Ar atmosphere containing 5% hydrogen, forming an NVP positive electrode layer and obtaining a sintered body. The NVP positive electrode layer of the resulting sintered body was masked with silicone rubber to form a circular exposed surface with a diameter of 7 mm. Sputtering was then performed on the exposed surface of the NVP positive electrode layer to form a current collector layer on only one side. The current collector layer was formed as a gold (Au) layer approximately 25 nm thick. A gold vapor deposition apparatus was used for sputtering. The sintered body was then transferred to a glove box under an argon atmosphere and sealed in an electrochemical measurement cell.

[0094] <Half-Cell Evaluation Method> (1) Charge-Discharge Cycle Test of Half-Cell The above-mentioned half-cell was connected to a multipotentio / galvanostat (SP-150e, manufactured by Biologic) so that the metallic Na side served as the reference electrode and counter electrode, and the negative electrode composite layer or NVP positive electrode layer side served as the working electrode, and measurements were performed at 25°C. Measurements of the negative electrode composite layer side were evaluated by repeating 80 cycles of charging to a cutoff voltage of 0 V at a current density corresponding to 0.1 C, resting for 10 hours, and discharging to a cutoff voltage of 3.1 V, and measuring the charge-discharge capacity for each cycle. Measurements of the NVP positive electrode layer side were evaluated by repeating 80 cycles of charging to a cutoff voltage of 4 V at a current density corresponding to 0.1 C, resting for 10 hours, and discharging to a cutoff voltage of 2.8 V, and measuring the charge-discharge capacity for each cycle. Figure 4 shows the evaluation results for a half-cell using HC, and Figure 5 shows the evaluation results for a half-cell using NVP. In FIGS. 4 and 5, the solid line indicates the measurement results at the first cycle, the dashed line indicates the measurement results at the tenth cycle, and the dashed dotted line indicates the measurement results at the 80th cycle.

[0095] (2) Evaluation Test of Charge / Discharge Rate Characteristics of Half Cell The above half cell was connected to a multipotentio / galvanostat (SP-150e, manufactured by Biologic) so that the metallic Na side served as the reference electrode and counter electrode, and the negative electrode composite layer or NVP positive electrode layer side served as the working electrode, and measurements were performed at 25 ° C. Measurements on the negative electrode composite layer side were performed by charging to a cutoff voltage of 0 V at a current density equivalent to 0.1 C, resting for 10 hours, and then discharging to a cutoff voltage of 3.1 V. Measurements were performed at current densities equivalent to 0.1 C, 0.2 C, 0.3 C, 0.4 C, and 0.5 C, and five cycles of charge / discharge were repeated, and the charge / discharge capacity of each cycle was measured and evaluated. Measurements on the NVP positive electrode layer side were performed by charging to a cutoff voltage of 4 V at a current density equivalent to 0.1 C, resting for 10 hours, and then discharging to a cutoff voltage of 2.8 V. The measurements were performed by repeating five cycles of charge and discharge at current densities corresponding to 0.05 C, 0.1 C, 0.2 C, 0.5 C, and 1.0 C, and measuring the charge and discharge capacity after each cycle. Figure 6 shows the evaluation results for the half cell using HC, and Figure 7 shows the evaluation results for the half cell using NVP. In Figures 6 and 7, filled squares indicate the measurement results for charge capacity, and open triangles indicate the measurement results for discharge capacity.

[0096] <Method for evaluating full cells> The full cells were connected to a multipotentio / galvanostat (SP-150e, manufactured by Biologic) so that the negative electrode composite layer side served as the reference electrode and counter electrode, and the NVP positive electrode layer side served as the working electrode, and measurements were performed at 25°C. Measurements were performed by repeating 40 cycles of charging to a cutoff voltage of 4.0 V at a current density equivalent to 0.1 C and discharging to a cutoff voltage of 1.5 V, and measuring the charge / discharge capacity for each cycle. FIG. 8 shows the evaluation results of the full cells using HC.

[0097] The present invention is not limited to the above-described embodiments, and encompasses various modifications and equivalent modifications within the scope of the spirit of the present invention. Therefore, in light of the above teachings, various combinations and forms, as well as other combinations and forms including only one element, more than one element, or less than one element, are to be understood as falling within the scope and spirit of the present invention.

Claims

1. A composition containing an oxide containing Na, Al, Zr and P as constituent elements, and a sintering aid.

2. The composition according to claim 1, wherein the oxide further contains Si as a constituent element.

3. The composition according to claim 1, wherein the sintering aid contains at least one element selected from the group consisting of Na, B, Bi, Zn, Nb and P.

4. The composition according to claim 1, wherein the sintering aid has a melting point of 1,000°C or less.

5. The composition according to claim 1, wherein the content of the sintering aid is 0.1 to 45.0 parts by mass per 100 parts by mass of the oxide.

6. The composition of claim 1, comprising two or more of said sintering aids.

7. The composition according to claim 1, comprising one or more types of sintering aids, the sintering aids containing Na and B as constituent elements in the same molecule, or Na and B in different molecules.

8. The composition according to any one of claims 1 to 7, which is for use as a solid electrolyte.

9. A fired product of the composition according to any one of claims 1 to 7.

10. The fired product according to claim 9, having a NASICON type crystal structure.

11. The fired product according to claim 10, having a crystal structure represented by the following formula (1): (In formula (1), M1 includes an element that becomes a divalent cation, M2 includes an element that becomes a trivalent cation other than Al, M3 includes an element that becomes a tetravalent cation (excluding Zr and Si), and a, b, c, d, and e satisfy "a≧0", "b>0", "c≧0", "d≧0", "0≦e<3", and "a+b+c+d<2".) 12. The fired product according to claim 11, wherein b≦0.5 is satisfied.

13. The fired product according to claim 11, wherein 1.5≦e≦2.8 is satisfied.

14. The fired product according to claim 9, having a relative density of 80% or more.

15. A solid electrolyte comprising the fired product according to claim 9.

16. An electricity storage device comprising the solid electrolyte according to claim 15.

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