JP2013168343A - Solid electrolyte, method for producing the same, and secondary battery provided with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain a high ionic conductivity even at a low temperature of less than 100 ° C. and to have a sufficiently high decomposition voltage and to be electrochemically stable, a method for producing the same, and a method for producing the same A secondary battery comprising:
A solid electrolyte comprising at least two kinds of sodium compounds selected from the group consisting of NaBH ₄ , NaNH ₂ and NaI.
[Selection figure] None

Description

  The present invention relates to a solid electrolyte, a manufacturing method thereof, and a secondary battery including the solid electrolyte.

  A lithium ion secondary battery is known as a battery having the highest energy density among the secondary batteries currently in practical use. In conventional lithium ion secondary batteries, a combustible organic solvent electrolysis is used as an electrolyte. Since a liquid is used, a safer solid electrolyte is required as an electrolyte of a secondary battery from the viewpoint of safety.

As a solid electrolyte including lithium, for example, International Publication No. 2009/139382 (Patent Document 1) discloses a solid electrolyte including LiBH ₄ and a specific alkali metal compound. However, lithium used for such an electrolyte has a small reserve and is unevenly distributed, and in a secondary battery using a conventionally known lithium ion electrolyte, the lithium nickel oxide ( Since lithium metal such as LiNiO ₂ ) or lithium cobaltate (LiCoO ₂ ) is used, there is a problem that a supply amount is insufficient and a problem that production cost is increased.

  Accordingly, solid electrolytes using sodium instead of lithium have been developed. For example, in Journal of Solid State Chemistry, 1980, Vol. 33, 385-390 (Non-patent Document 1), A sodium ion solid electrolyte is disclosed, and Solid State Ionics, 1981, 3-4, 215-218 (Non-patent Document 2) discloses a NASICON-based sodium ion solid electrolyte. However, compared to lithium, there are currently few known substances that can be used as a material for a sodium ion solid electrolyte, and there are only a few examples of using sodium for a solid electrolyte. Also in the electrolyte, the electrical stability that the structure and the ionic conductivity are maintained under a high voltage is not sufficient.

International Publication No. 2009/139382

I. L. BRANT, G. C. FARRINGTON, Journal of Solid State Chemistry, 1980, 33, 385-390 U. VON ALPEN, M.M. F. BELL, H.C. H. HOFER, Solid State Ionics, 1981, 3-4, 215-218

  The present invention has been made in view of the above-mentioned problems of the prior art, and can maintain high ionic conductivity even at a low temperature of less than 100 ° C., and has a sufficiently high decomposition voltage and is electrochemical. It is an object to provide a highly stable sodium ion solid electrolyte, a method for producing the same, and a secondary battery including the solid electrolyte.

As a result of intensive studies to achieve the above object, the present inventors have used sodium compounds (sodium complex hydrides) such as NaBH ₄ , NaNH ₂ and NaI alone as the solid electrolyte material. The ionic conductivity is remarkably lowered as the temperature is lowered, whereas at least two kinds of sodium compounds selected from the group consisting of NaBH ₄ , NaNH ₂ and NaI are mixed in combination and heated. It has been found that the solid electrolyte obtained by melting or sintering can maintain a high sodium ion conductivity even at a low temperature of less than 100 ° C. and has a sufficiently high decomposition voltage. In addition, such a solid electrolyte maintains its structure and excellent ionic conductivity even at a high voltage of 6 V or higher, and further maintains high ionic conductivity over time, so that it is also electrochemically stable. As a result, the present invention has been completed.

That is, the solid electrolyte of the present invention is characterized by comprising at least two kinds of sodium compounds selected from the group consisting of NaBH ₄ , NaNH ₂ and NaI.

The solid electrolyte of the present invention, preferably comprise a NaBH ₄ and NaNH _2, NaBH ₄ and NaNH ₂ molar ratio NaBH with _{4: NaNH 2 = 1: 7~7} : and more preferably 1.

Further, as the solid electrolyte of the present invention, when combined with NaBH ₄ and NaNH _2, in the Raman spectrum measurement, at least, preferably has a peak at two positions 2300 ± 100 cm ^-1 and 3200 ± 100 cm ^-1 In addition, in X-ray diffraction measurement (CuKα: λ = 1.5405４０), it is preferable to have diffraction peaks at least at three locations of 2θ = 19 ± 1 deg, 33 ± 1 deg, 38.5 ± 1 deg.

Moreover, the secondary battery of this invention is equipped with the solid electrolyte of the said invention. Furthermore, in the method for producing a solid electrolyte of the present invention, at least two kinds of sodium compounds selected from the group consisting of NaBH ₄ , NaNH ₂ and NaI are mixed, and the mixture is heated to melt or sinter. It is a feature.

  As a method for producing the solid electrolyte of the present invention, it is preferable to subject the mixture to a mixing and pulverizing treatment using a ball mill.

Furthermore, in the method for producing a solid electrolyte of the present invention, NaBH ₄ and NaNH ₂ are preferably used as the sodium compound, and the molar ratio of NaBH ₄ and NaNH ₂ is NaBH ₄ : NaNH ₂ = 1: 7-7. : 1 is more preferable.

  According to the present invention, a high ion conductivity can be maintained even at a low temperature of less than 100 ° C., and a sufficiently high decomposition voltage and an electrochemically stable sodium ion solid electrolyte, a method for producing the same, and A secondary battery including the solid electrolyte can be provided.

It is a model perspective view which shows the anti-perovskite structure in this invention. It is a graph which shows the X-ray-diffraction pattern of the solid electrolyte obtained in Examples 1-3 and Comparative Examples 1-2. It is a graph which shows the Raman spectrum of the solid electrolyte obtained in Example 2 and Comparative Examples 1-2. It is a graph which shows the X-ray-diffraction pattern of the solid electrolyte obtained in Examples 4-6 and Comparative Examples 1 and 3. FIG. It is a graph which shows the X-ray-diffraction pattern of the solid electrolyte obtained in Examples 7-9 and Comparative Examples 2-3. It is a graph which shows the relationship between the ionic conductivity of the solid electrolyte membrane obtained by Example 2 and Comparative Examples 1-2, and measurement temperature. It is a graph which shows the relationship between the ionic conductivity of the solid electrolyte membrane obtained by Example 5 and Comparative Examples 1 and 3, and measurement temperature. It is a graph which shows the relationship between the ionic conductivity of the solid electrolyte membrane obtained in Example 8 and Comparative Examples 2-3, and measurement temperature. It is a graph which shows the relationship between the electrical conductivity at the time of using a sodium electrode about the solid electrolyte membrane obtained in Example 2, and a molybdenum electrode, and measurement time. It is a graph which shows the relationship between the electric current at the time of implementing decomposition voltage measurement about the solid electrolyte membrane obtained in Example 2. FIG. It is a graph which shows the X-ray-diffraction pattern before and after implementing decomposition voltage measurement about the solid electrolyte membrane obtained in Example 2, and the X-ray-diffraction pattern of the solid electrolyte obtained in Comparative Examples 1-2.

Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof. First, the solid electrolyte of the present invention will be described. The solid electrolyte of the present invention comprises at least two kinds of sodium compounds selected from the group consisting of NaBH ₄ , NaNH ₂ and NaI.

The NaBH 4 according to the present invention _(sodium borohydride) is not particularly limited, may be used conventionally known which are used as the reducing agent and the hydrogen storage medium. The purity of NaBH ₄ is preferably 90% or more, and more preferably 95% or more. When the purity of NaBH ₄ is less than the lower limit, ionic conductivity and electrochemical stability tend to decrease.

The NaNH 2 _(sodium amide) according to the present invention is not particularly limited, may be used conventionally known which is used as a co-base. The purity of the NaNH ₂ is preferably 90% or more, and more preferably 95% or more. When the purity of NaNH ₂ is less than the lower limit, the ionic conductivity and the electrochemical stability tend to decrease.

  There is no restriction | limiting in particular as NaI (sodium iodide) based on this invention, A conventionally well-known thing can be used. The purity of the NaI is preferably 90% or more, and more preferably 95% or more. When the purity of NaI is less than the lower limit, the ionic conductivity and electrochemical stability tend to decrease.

The solid electrolyte of the present invention comprises at least two kinds of sodium compounds selected from the group consisting of NaBH ₄ , NaNH ₂ and NaI. As the combination of the sodium compounds, NaBH ₄ , NaNH ₂ and NaI (Na (BH ₄ ) —Na (NH ₂ ) —NaI), NaBH ₄ and NaNH ₂ (Na (BH ₄ ) — Na (NH ₂ )), NaBH ₄ and NaI (Na (BH ₄ ) -NaI), NaNH ₂ and NaI (Na (NH ₂ ) -NaI) From the viewpoint that conductivity and decomposition voltage can be further increased and higher ionic conductivity can be maintained, a combination of NaBH ₄ and NaNH ₂ (Na (BH ₄ ) -Na (NH ₂ )) is preferable. .

In the solid electrolyte of the present _invention, NaBH _4, NaNH ₂ and when combining NaI, As the content molar _ratio, NaBH _4, NaNH ₂ and NaBH ₄ NaI on the diagram pseudo ternary phase that an apex: NaI = 9 1: NaNH ₂ : NaI = 1: 1, NaBH ₄ : NaNH ₂ = 1: 7 to 7: 1, preferably in the composition region, NaBH ₄ : NaI = 9: 1, NaNH ₂ : More preferably, it is within the composition region connected by NaI = 1: 1, NaBH ₄ : NaNH ₂ = 1: 3 to 3: 1, NaBH ₄ : NaI = 9: 1, NaNH ₂ : NaI = 1: 1 More preferably, it is in the composition region connected by NaBH ₄ : NaNH ₂ = 1: 2 to 2: 1. When the content molar ratio of NaBH ₄ , NaNH ₂ and NaI is less than the lower limit and exceeds the upper limit, respectively, the ionic conductivity and the electrochemical stability tend to decrease.

In the solid electrolyte of the present invention, when NaBH ₄ and NaNH ₂ are combined, the content molar ratio (NaBH ₄ : NaNH ₂ ) is preferably 1: 7 to 7: 1, and 1: 3 to 3: 1. Is more preferably 1: 2 to 2: 1, and particularly preferably 1: 1. When the content molar ratio of NaBH ₄ is less than the above lower limit and exceeds the above upper limit, it becomes difficult to become a single phase of the antiperovskite structure described later, and the ionic conductivity and electrochemical stability. Tend to decrease.

In the solid electrolyte of the present invention, when NaBH ₄ and NaI are combined, the molar ratio (NaBH ₄ : NaI) is preferably 1: 7 to 9: 1, and preferably 1: 3 to 9: 1. It is more preferable that the ratio is 1: 2 to 9: 1. When the content molar ratio of NaBH ₄ is less than the lower limit and exceeds the upper limit, the ionic conductivity and electrochemical stability tend to decrease.

In the solid electrolyte of the present invention, when NaNH ₂ and NaI are combined, the molar ratio (NaNH ₂ : NaI) is preferably 1: 7 to 7: 1, and preferably 1: 3 to 3: 1. It is more preferable that the ratio is 1: 1. In both cases where the molar ratio of NaNH ₂ is less than the lower limit and exceeds the upper limit, ionic conductivity and electrochemical stability tend to decrease.

Further, in such a solid electrolyte, for combining the NaBH ₄ and NaNH ₂ is Raman spectrum measured (wavelength: 532 nm) at, at least, the peak at two positions 2300 ± 100 cm ^-1 and 3200 ± 100 cm ^-1 preferably has at least, 1300 ± ^{100 cm} -1, more preferably has a peak at three positions of 2300 ± 100 cm ^-1 and 3200 ± 100 cm ^-1, at ^{least, 1300 ± 100cm -1, 1500 ±} 100cm -1, More preferably, it has peaks at four locations of 2300 ± 100 cm ⁻¹ and 3200 ± 100 cm ⁻¹ . These peaks are derived from BH ₄ ^— and NH ₂ ^— . In the present invention, the presence of BH ₄ and NH ₂ as one element results in an antiperovskite structure described later. Therefore, the ionic conductivity (sodium ionic conductivity) tends to be further improved.

Further, when NaBH ₄ and NaNH ₂ are combined in the solid electrolyte, at least 2θ = 19 ± 1 deg, 33 ± 1 deg, 38.5 ± 1 deg in X-ray diffraction (CuKα: λ = 1.5405 mm). It is preferable to have diffraction peaks at three locations, more preferably at least 2θ = 19 ± 1 deg, 33 ± 1 deg, 38.5 ± 1 deg, and 43 ± 1 deg, and at least 2θ = 19. It is more preferable to have diffraction peaks at eight locations of ± 1 deg, 27 ± 1 deg, 33 ± 1 deg, 38.5 ± 1 deg, 43 ± 1 deg, 47.5 ± 1 deg, 55.5 ± 1 deg, and 59 ± 1 deg. These diffraction peaks are derived from the antiperovskite structure.

In the present invention, the antiperovskite structure is a perovskite structure of a compound (ABX ₃ ) composed of a normal ternary system (a cation A ^{+ at} the apex of a cubic crystal, a cation B ^{2+ at} the body ^center , and an anion at the face ^center) . X ^- is a structure as it were reversed from the structure) which is arranged, when combined with NaBH ₄ and NaNH ₂ in the present invention, as shown schematically in Figure 1, BH ₄ to the vertex ^- a body-centered Shows a structure in which NH ₂ ⁻ , Na ⁺ is arranged in the center, and one Na ⁺ is a vacancy. In general, the present inventors have found that NaBH ₄ and NaNH ₂ have extremely low ionic conductivity at a low temperature of less than 100 ° C. and are difficult to use as a solid electrolyte. On the other hand, the present inventors have also found that the solid electrolyte obtained by combining NaBH ₄ and NaNH ₂ in the present invention has such an anti-resistance even at a low temperature (less than 100 ° C. (more preferably less than 50 ° C.)). It has been found that by having a perovskite structure, the ionic conductivity and the decomposition voltage are further increased, a higher ionic conductivity is maintained, and more excellent electrochemical stability is exhibited.

The reason why the ionic conductivity and the decomposition voltage are further increased and the higher ionic conductivity is maintained by having such an antiperovskite structure is not necessarily clear, but the present inventors are as follows. To guess. That is, in the present invention, in the case of the antiperovskite structure as described above, since one Na ⁺ is a vacancy, sodium ions can also move through this vacancy, and ion conduction The present inventors infer that the degree of ionic conductivity is further improved and higher ionic conductivity is maintained even in a low temperature region.

Next, the manufacturing method of the solid electrolyte of this invention is demonstrated. The solid electrolyte of the present invention is manufactured by mixing at least two kinds of sodium compounds selected from the group consisting of NaBH ₄ , NaNH ₂ and NaI, and heating and melting or sintering the obtained mixture. Can do. The method for producing the solid electrolyte of the present invention is not particularly limited to this method, but from the viewpoint that a solid electrolyte uniformly including two or more kinds of sodium compounds can be obtained, the obtained mixture is heated and sintered. The method of ligating is preferred. The mixed molar ratio of NaBH ₄ , NaNH ₂ and NaI is the same as the molar ratio contained in the solid electrolyte of the present invention.

As a method for producing a solid electrolyte of the present invention, it is possible to obtain a solid electrolyte having more uniform two or more kinds of sodium compounds and more excellent ion conductivity. In particular, in the present invention, NaBH ₄ and NaNH ₂ are used. In the case of combining, it is preferable to subject the mixture to a mixing and pulverizing treatment before the melting or sintering from the viewpoint that the antiperovskite structure can be obtained. As the mixing and pulverizing method, a known method may be used as appropriate, and examples thereof include a method using a pulverizer such as a ball mill and a jet mill. As the production method of the present invention, it is more preferable to subject the mixture to a mixing and pulverizing treatment by a ball mill from the viewpoint that a solid electrolyte having superior properties can be obtained from the same viewpoint as described above.

Examples of the ball mill method include a method of obtaining a pulverized mixture by putting the mixture in a container together with balls for mixing and pulverizing and rotating the container. In such a ball mill, it is preferable to perform ball milling so that the average particle size of the powder of the pulverized mixture is 1 to 500 μm (more preferably 10 to 100 μm). When the average particle size of the powder is less than the lower limit, the cost and time for mixing tend to increase. On the other hand, when the upper limit is exceeded, the mixture is not sufficiently mixed and ion conduction is caused. The degree and decomposition voltage tend to decrease, and when NaBH ₄ and NaNH ₂ are combined, it tends to be difficult to obtain a single phase of the antiperovskite structure. In addition, such an average particle diameter can be measured using an X-ray diffractometer, a scanning electron microscope (SEM), a particle measuring device, or the like.

  The material of the balls for mixing and grinding is not particularly limited, and examples thereof include iron, zirconia, and alumina. In addition, since the size of such a ball varies depending on the capacity of the container to be used, etc., it cannot be generally stated, but when the capacity of the container is 10 to 500 mL (more preferably 30 to 100 mL), From the viewpoint of keeping the average particle size of the powder of the pulverized mixture within the above range, it is preferable to use a powder having a diameter of 1 to 10 mm (more preferably 4 to 8 mm). Furthermore, from the viewpoint of keeping the average particle size of the powder of the pulverized mixture within the above range, the rotation speed in the ball mill is preferably 50 to 800 rpm (more preferably 300 to 500 rpm), and the time is It is preferably 10 to 1200 minutes (more preferably 30 to 300 minutes).

  In the melting and sintering, the heating temperature is usually 100 ° C. or higher, preferably 150 ° C. or higher, and more preferably 150 to 400 ° C. When the heating temperature is less than the lower limit, the mixture tends to be insufficiently melted or sintered. On the other hand, when the heating temperature exceeds the upper limit, the compound in the mixture evaporates or is converted to other compounds. It tends to decompose.

  In the present invention, when the solid electrolyte is formed into a predetermined shape, the mixture (preferably a pulverized mixture) may be preliminarily molded and then heated, or may be molded after heating. For example, after the mixture is formed into a predetermined shape by press molding or the like, it is heated and melted or sintered while maintaining this shape, and then cooled. In addition, the mixture is heated and melted, the melted mixture is molded into a predetermined shape and cooled, or the mixture is heated and sintered, and the sintered product is cooled and then molded into a predetermined shape. May be.

  Next, the secondary battery of the present invention will be described. The secondary battery of the present invention comprises the solid electrolyte of the present invention, a positive electrode, and a negative electrode. Examples of the positive electrode material include compounds containing sodium such as sodium cobaltate and sodium chromite, and examples of the negative electrode material include compounds containing sodium such as sodium and sodium sulfide; and carbon materials.

  In the secondary battery of the present invention, for example, the positive electrode material is applied to one surface of a solid electrolyte formed into a film to form a positive electrode, and the negative electrode material is applied to the other surface to form a negative electrode. Alternatively, the film-like positive electrode material can be attached to one surface of a solid electrolyte formed into a film shape, and the film-like negative electrode material can be attached to the other surface.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example. The structural analysis, ion conductivity measurement, ion transport number measurement, and decomposition voltage measurement in each example and comparative example were performed by the following methods. Further, each operation in Examples and Comparative Examples was performed under an argon gas atmosphere (oxygen concentration 0.1 ppm or less, dew point −93 ° C. (180 K) or less) unless otherwise specified.

(Structural analysis)
<X-ray diffraction measurement>
For the solid electrolytes obtained in each Example and Comparative Example, the following equipment and measurement conditions are shown below:
Device: X'PERT-Pro (manufactured by PANalytical)
X-ray: CuKα (λ = 1.5405mm)
Temperature: 23 ° C (296K)
Measurement was carried out by The lattice constant was determined using analysis programs “TREOR97” and “PRUM”.
<Raman spectroscopy measurement>
For the solid electrolytes obtained in each Example and Comparative Example, the following equipment and measurement conditions are shown below:
Device: Almega-HD (Nicolet)
Wavelength: 532nm
Measurement time: 60 seconds Temperature: 23 ° C (296K)
Measurement was carried out by

(Ion conductivity measurement)
Molybdenum foil was attached to both surfaces of the solid electrolyte membranes obtained in each Example and Comparative Example, and incorporated in an electrochemical measurement cell manufactured by Hosen. While this electrochemical measurement cell was heated from 27 ° C. (300 K) to 150 ° C. (423 K) with a measurement time of about 5 minutes per point at about 10 ° C., a “chemical impedance meter 3532-80” manufactured by Hioki Electric Co., Ltd. The ionic conductivity (Scm ⁻¹ ) of the solid electrolyte membrane was measured by an alternating current method (frequency 4 Hz to 1 MHz).

(Ion transport number measurement)
The solid electrolyte membranes obtained in each of the examples and comparative examples were fitted with sodium foil or molybdenum foil into an electrochemical measurement cell manufactured by Hosen Co., Ltd. An electrochemical measurement cell was prepared. For each electrochemical measurement cell, at 60 ° C. (333 K), “VersaSTAT4” manufactured by Princeton was used, and the electrical conductivity (Scm ⁻¹ ) change with time (0 to 300 seconds) by the direct current method (0.1 V). The ion transport number was measured by measuring.

(Measurement of decomposition voltage)
The solid electrolyte membranes obtained in each Example and Comparative Example were each fitted with a sodium foil on one side and a molybdenum foil on the other side into an electrochemical measurement cell manufactured by Hosen. With respect to this electrochemical measurement cell, the current (μA) was measured by cyclic voltammetry at 60 ° C. (333 K) using “VersaSTAT4” manufactured by Princeton in a range of ⁻¹ to 6 V under a scanning speed of ¹ mVs ^−1. The decomposition voltage (V) was measured by measuring.

Example 1
NaBH ₄ (purity 98%) and NaNH ₂ (purity 95%) were weighed so as to have a molar ratio of NaBH ₄ : NaNH ₂ = 1: 3 and used together with iron balls (20 pieces) having a diameter of 7 mm. Put into a container (30 mL), and ball mill by rotating for 30 minutes at a temperature of 23 ° C. and 400 rpm using a ball mill apparatus (trade name: Planetary Ball Mill P-7, manufactured by Fritsch) in an argon gas atmosphere. (Average particle size: 10 μm). Subsequently, this powder was transferred to a molybdenum container and then sintered at a temperature of 150 ° C. for 12 hours in a 0.2 MPa hydrogen gas atmosphere. Thereafter, the sintered product was cooled to room temperature (23 ° C.) to obtain a solid electrolyte.

  In addition, the sintered product obtained in the same manner as described above was press-molded (25 ° C., 20 MPa) into a disc shape (diameter 8 mm, thickness 2 mm) to obtain a solid electrolyte membrane.

(Example 2)
A solid electrolyte and a solid electrolyte membrane were obtained in the same manner as in Example 1 except that the molar ratio of NaBH ₄ and NaNH ₂ (NaBH ₄ : NaNH ₂ ) was 1: 1.

(Example 3)
A solid electrolyte and a solid electrolyte membrane were obtained in the same manner as in Example 1 except that the molar ratio of NaBH ₄ and NaNH ₂ (NaBH ₄ : NaNH ₂ ) was 3: 1.

Example 4
Using NaI (purity 99.999%) in place of NaNH _2, the molar ratio of NaBH ₄ and NaI _(NaBH 4: NaI) is 1: set to be 3, the sintering temperature except for the 400 ° C. A solid electrolyte and a solid electrolyte membrane were obtained in the same manner as in Example 1.

(Example 5)
A solid electrolyte and a solid electrolyte membrane were obtained in the same manner as in Example 4 except that the molar ratio of NaBH ₄ and NaI (NaBH ₄ : NaI) was 1: 1.

(Example 6)
A solid electrolyte and a solid electrolyte membrane were obtained in the same manner as in Example 4 except that the molar ratio of NaBH ₄ and NaI (NaBH ₄ : NaI) was 3: 1.

(Example 7)
Using NaI instead of NaBH _4, the molar ratio of NaNH ₂ and NaI _(NaNH 2: NaI) is 1: except that as the 3 in the same manner as in Example 1 to obtain a solid electrolyte and a solid electrolyte membrane It was.

(Example 8)
A solid electrolyte and a solid electrolyte membrane were obtained in the same manner as in Example 7 except that the molar ratio of NaNH ₂ to NaI (NaNH ₂ : NaI) was 1: 1.

Example 9
A solid electrolyte and a solid electrolyte membrane were obtained in the same manner as in Example 7 except that the molar ratio of NaNH ₂ to NaI (NaNH ₂ : NaI) was 3: 1.

(Comparative Example 1)
A solid electrolyte (NaBH ₄ ) and a solid electrolyte membrane (NaBH ₄ ) were obtained in the same manner as in Example 1 except that NaNH ₂ was not used.

(Comparative Example 2)
A solid electrolyte (NaNH ₂ ) and a solid electrolyte membrane (NaNH ₂ ) were obtained in the same manner as in Example 1 except that NaBH ₄ was not used.

(Comparative Example 3)
A solid electrolyte (NaI) and a solid electrolyte membrane (NaI) were obtained in the same manner as in Example 1 except that NaI was used in place of NaBH ₄ and NaNH ₂ .

Example 1-3 obtained in the solid electrolyte _{(Na (BH 4) -Na (} NH 2)) result of the X-ray diffractometry shown in FIG. Although FIG. 2 also shows the X-ray diffraction spectrum of Comparative Example 1 (NaBH ₄₎ and Comparative Example 2 (NaNH _2).

  As is clear from the results shown in FIG. 2, in the solid electrolyte obtained in Example 1, the positions of 2θ = 19 ± 1 deg, 27 ± 1 deg, 33 ± 1 deg, and 38.5 ± 1 deg. In the obtained solid electrolyte, 2θ = 19 ± 1 deg, 27 ± 1 deg, 33 ± 1 deg, 38.5 ± 1 deg, 43 ± 1 deg, 47.5 ± 1 deg, 55.5 ± 1 deg, and 59 ± 1 deg, In the solid electrolyte obtained in Example 3, X-ray diffraction peaks were observed at positions of 2θ = 19 ± 1 deg, 33 ± 1 deg, and 38.5 ± 1 deg, respectively, and the lattice constant was 0 in all the examples. 46909 nm. From these results, it was confirmed that all of the solid electrolytes obtained in Examples 1 to 3 had an antiperovskite structure.

Also shows the result of the Raman spectrum measured for the solid electrolyte obtained in Example _{2 (Na (BH 4) -Na} (NH 2)) in FIG. FIG. 3 also shows the Raman spectra of Comparative Example 1 (NaBH ₄ ) and Comparative Example 2 (NaNH ₂ ). As shown in FIG. ^3, 2310cm ^-1, a peak is observed at three positions on the 3230Cm ^-1 and 3270cm ^-1, the solid electrolyte of the present invention is BH ₄ ^- to be anti-perovskite structure having ^- and NH ₂ confirmed.

The result of the X-ray diffraction measurement for the obtained solid electrolyte in Example _{4~6 (Na (BH 4) -NaI} ) shown in FIG. FIG. 4 also shows the X-ray diffraction spectra of Comparative Example 1 (NaBH ₄ ) and Comparative Example 3 (NaI). From this, it was confirmed that all the solid electrolytes obtained in Examples 4 to 6 have a sodium chloride type (Fm-3m) structure.

The result of the X-ray diffraction measurement for the obtained solid electrolyte in Example _{7~9 (Na (NH 2) -NaI} ) shown in FIG. FIG. 5 also shows the X-ray diffraction spectra of Comparative Example 2 (NaNH ₂ ) and Comparative Example 3 (NaI). This confirmed that all of the solid electrolytes obtained in Examples 7 to 9 had a silver azide type (Ibam) structure.

  The ionic conductivity was measured for each of the solid electrolyte membranes obtained in the examples and comparative examples. The results obtained in Example 2 and Comparative Examples 1 and 2 are obtained in FIG. 6, the results obtained in Example 5 and Comparative Examples 1 and 3 are obtained in FIG. 7, and the results obtained in Example 8 and Comparative Examples 2 and 3 are obtained. The results are shown in FIG.

As is apparent from the results shown in FIGS. 6 to 8, in the case of the solid electrolyte having only NaBH ₄ , NaNH ₂ , and NaI (Comparative Examples 1 to 3), the ionic conduction is extremely high particularly at a low temperature (less than 50 ° C.). relative degrees of low ^{(27 ℃ 2 × 10 -10 Scm} -1 or less at), in the case of the solid electrolyte of the present invention has a high ionic conductivity even at a low temperature is maintained, has sufficient ionic conductivity It was confirmed. In particular, when NaBH ₄ and NaNH ₂ are combined (Example 2), the ion conductivity is 2 × 10 ⁻⁶ Scm ⁻¹ at 27 ° C., which is 10,000 times or more that of Comparative Examples 1 and 2. It was confirmed that

  The result of having carried out the ion transport number measurement about the solid electrolyte membrane obtained in Example 2 is shown in FIG. Since the electrical conductivity is 40,000 times greater when sodium is used as the electrode than when molybdenum is used as the electrode, the solid electrolyte of the present invention is a sodium ion conductor having an ion transport number of approximately 1, and is sufficiently It was confirmed to have a good sodium ion conductivity.

  Moreover, the graph which shows the relationship between the electric current (microampere) at the time of implementing a decomposition voltage measurement about Example 2, and a voltage (V) is shown in FIG. From this, the measurement of the decomposition voltage of the solid electrolyte membrane obtained in Example 2 is 6.0 V or higher, and it is confirmed that the decomposition voltage is sufficiently high, and is useful as a solid electrolyte of a secondary battery. It was confirmed. Moreover, the result of having implemented the X-ray-diffraction measurement before and after the decomposition voltage measurement in Example 2 is shown in FIG. 11 with the result of Comparative Examples 1-2. From this, even after the decomposition voltage measurement, the structure before the decomposition voltage measurement was maintained, and it was confirmed that the solid electrolyte of the present invention was electrochemically stable.

  As described above, according to the present invention, a high ion conductivity can be maintained even at a low temperature of less than 100 ° C., and a sufficiently high decomposition voltage and an electrochemically stable sodium ion solid electrolyte. , Its manufacturing method, and its solid electrolyte can be provided. Therefore, the solid electrolyte of the present invention is very useful as a solid electrolyte for sodium ion secondary batteries.

1: BH ₄ ⁻ , 2: NH ₂ ⁻ , 3: Na ⁺ , 4: vacancy.

Claims (10)

A solid electrolyte comprising at least two kinds of sodium compounds selected from the group consisting of NaBH ₄ , NaNH ₂ and NaI. The solid electrolyte according to claim 1, comprising NaBH ₄ and NaNH ₂ . NaBH ₄ and NaNH ₂ molar ratio NaBH with _{4: NaNH 2 = 1: 7~7} : solid electrolyte according to claim 2, characterized in that it is 1. In the Raman spectrum measurement, at least, a solid electrolyte according to claim 2 or 3 characterized by having a peak at two positions 2300 ± 100 cm ^-1 and 3200 ± 100 cm ^-1.   5. An X-ray diffraction measurement (CuKα: λ = 1.5405Å) has diffraction peaks at least at three positions of 2θ = 19 ± 1 deg, 33 ± 1 deg, 38.5 ± 1 deg. The solid electrolyte according to any one of the above.   A secondary battery comprising the solid electrolyte according to claim 1. A method for producing a solid electrolyte, comprising mixing at least two kinds of sodium compounds selected from the group consisting of NaBH ₄ , NaNH ₂ and NaI, and heating or melting or sintering the mixture.   The method for producing a solid electrolyte according to claim 7, wherein the mixture is subjected to a mixing and pulverizing process using a ball mill. Method for producing a solid electrolyte according to claim 7 or 8, characterized by using the NaBH ₄ and NaNH ₂ as the sodium compound. NaBH ₄ and NaNH ₂ molar ratio NaBH with _{4: NaNH 2 = 1: 7~7} : method for producing a solid electrolyte according to claim 9, characterized in that it is 1.