JP6762861B2 - Sulfide-based solid electrolyte and sodium battery - Google Patents

Description

The present invention relates to a sulfide-based solid electrolyte and a sodium battery using the same.

Sodium secondary batteries are expected as low-cost next-generation batteries against the background of sodium, which is a low environmental load material. Currently, in sodium-sulfur batteries (NaS batteries) used for day and night load leveling for large-scale power storage, β-alumina crystals are used as solid electrolytes, but the sodium ion conductivity of solid electrolytes is used. In order to ensure, the operating temperature is limited to a high temperature of 300 ° C. or higher.

Against this background, by using a sulfide-based solid electrolyte having a composition of Na ₃ PS ₄ with a high conductivity of 10 ^-4 S / cm at room temperature, sodium that has operated only at high temperatures so far -It has been reported that a sulfur battery can be operated at room temperature (see, for example, Non-Patent Document 1). Further, a sodium conductor having a high conductivity of 7 × 10 ^-4 S / cm has been proposed (see, for example, Patent Document 1).
It has also been reported that the conductivity of a solid electrolyte for a lithium secondary battery, which is also expected as a power storage battery, is about ^10-2 S / cm, which is about an order of magnitude higher (see, for example, Non-Patent Document 2).

An object of the present invention is to provide a sulfide-based solid electrolyte that can obtain a high conductivity equal to that of a solid electrolyte for a lithium secondary battery.

The sulfide-based solid electrolyte of the present invention as a means for solving the above-mentioned problems is a sulfide-based solid electrolyte containing Na element, P element, and S element and having a composition of Na ₃ PS ₄ .
The crystal structure of Na ₃ PS ₄ is tetragonal.
The lattice constant a of Na ₃ PS ₄ is 6.948Å or more and 6.970Å or less.
The lattice constant c of Na ₃ PS ₄ is 7.087 Å or more and 7.096 Å or less.

According to the present invention, it is possible to provide a sulfide-based solid electrolyte for a sodium secondary battery, which can obtain a high conductivity at the same level as the conductivity of the solid electrolyte for a lithium secondary battery.

FIG. 1 is an X-ray diffraction pattern of the sulfide-based solid electrolytes of Examples 1 to 4 and Comparative Examples 1 and 2. FIG. 2 is a diagram showing the relationship between the lattice constant a obtained by Rietveld analysis from the X-ray diffraction diagram of FIG. 1 and the raw material charging composition for Examples 1 to 4. FIG. 3 is a diagram showing the relationship between the lattice constant c obtained by Rietveld analysis from the X-ray diffraction diagram of FIG. 1 and the raw material charging composition for Examples 1 to 4. FIG. 4 is a diagram showing the relationship between the conductivity and the raw material charging composition for Examples 1 to 4. FIG. 5 is a diagram showing a transition of the discharge capacity of the sodium secondary battery of Example 5 when the charge / discharge cycle is repeated. FIG. 6 is a diagram showing the crystal structure of tetragonal Na ₃ PS ₄ without Na defects. FIG. 7 is a diagram showing the crystal structure of the tetragonal Na ₃ PS ₄ having the Na defect of the present invention.

(Sulfide-based solid electrolyte)
The sulfide-based solid electrolyte of the present invention is a sulfide-based solid electrolyte containing Na element, P element, and S element and having a composition of Na ₃ PS ₄ .
The crystal structure of Na ₃ PS ₄ is tetragonal.
The lattice constant a of Na ₃ PS ₄ is 6.948Å or more and 6.970Å or less.
The lattice constant c of Na ₃ PS ₄ is 7.087 Å or more and 7.096 Å or less.
Further, it is preferable that Na of Na ₃ PS ₄ has a defect.

The lattice constant a of the known tetragonal Na ₃ PS ₄ is 6.9520 Å, and the lattice constant c is 7.0757 Å (M. Jansen and U. Henseler, Synthesis, Structure Determination, and Ionic Synthesis, and Ionic Synthesis). OF SOLID STATE CHEMISTRY, 99, 110-119 (1992)).

The sulfide-based solid electrolyte of the present invention is based on the finding that, in the prior art, no solid electrolyte for a sodium secondary battery having the same conductivity as that of a solid electrolyte for a lithium secondary battery has been obtained. ..
As a result of diligent studies by the present inventors, they have found that the above problems can be solved by setting the lattice constant a and the lattice constant c of the tetragonal Na ₃ PS ₄ to the optimum range, and have completed the present invention.
Thus, the sulfide-based solid electrolyte of the present invention comprises a Na element, P element, and S elements, and has the composition _Na 3 PS _4, the crystal structure of the _Na 3 PS ₄ is a tetragonal, the The lattice constant a of Na ₃ PS ₄ is 6.948 Å or more and 6.970 Å or less, and the lattice constant c of Na ₃ PS ₄ is 7.087 Å or more and 7.096 Å or less, more preferably the Na ₃ PS ₄ Since Na has a defect, a high conductivity equal to that of the solid electrolyte for a lithium secondary battery can be obtained, which is suitable as a solid electrolyte for a sodium secondary battery.

Regarding the above-mentioned lattice constant range, if it is smaller than the specified range, the ions become difficult to move due to steric hindrance and the conductivity drops. On the contrary, if it is larger than the specified range, ion hopping when the ions are conducted does not proceed smoothly. In that case, the conductivity drops. That is, in the present invention, this proper grid size is important. Furthermore, Na defects in the crystal also contribute to conductivity. The Na defect of the present invention corresponds to a Frenkel defect, and is a defect in which lattice point ions are transferred between lattices and pores remain after that. Regarding this, FIG. 6 shows a tetragonal Na ₃ PS ₄ without defects, and FIG. 7 shows a structure in which Na moves due to defects. Regarding the movement of Na in FIG. 7, Na1 moves to Na4 and Na2 moves to Na3. The presence of a gap due to such a defect facilitates the movement of ions, leading to an improvement in the conductivity of the present invention. Further, it is considered that Na (Na3) moved between the lattices is connected in the ion conductive direction, that is, in the c-axis direction to form an ion conductive path, which also contributes to the improvement of the conductivity.

Here, the Rietveld method used when determining the occupancy (deficiency) of the lattice constants a, c, and Na will be described.
The Rietveld method is a structural analysis method that makes it possible to obtain many highly accurate structural parameters from diffraction figures obtained by powder X-ray diffraction measurement or neutron diffraction measurement. In the analysis, the diffraction points obtained by the measurement are fitted with the integrated intensity and peak shape calculated from the predicted structural model, and the diffraction points calculated by approximating the profile to refine the structure.
Also in the present invention, the Rietveld method can be used when determining the occupancy (deficiency) of the lattice constants a, c, and Na, and Z-Rietveld can be used for the analysis program.

The sulfide-based solid electrolyte is not particularly limited and may be appropriately selected depending on the intended purpose. However, in the powder X-ray diffraction measurement using Cu-Kα rays having an X-ray wavelength of 1.5418 angstroms (Å), 17. It is preferable to have a characteristic peak near the diffraction angles (2θ) of 8 °, 18.0 °, 31.0 °, 31.3 °, 36.1 ° and 36.5 °.
Here, FIG. 1 shows a powder X-ray diffraction pattern of the sulfide-based solid electrolytes of Examples 1 to 4 and Comparative Examples 1 and 2 described later. The structure of Na ₃ PS ₄ (tetragonal or cubic) can be identified from this diffraction pattern.

The starting material for synthesizing the sulfide-based solid electrolyte is not particularly limited and may be appropriately selected depending on the purpose, for example, sodium, sulfur, phosphorus, sodium sulfide (Na 2 _S), phosphorus sulfide (P ₄ S ₃ , P ₂ S ₅ , P ₄ S ₇ ) and the like. One of these may be used alone, or two or more thereof may be used in combination. Of these, sodium sulfide (Na ₂ S) and diphosphorus pentasulfide (P ₂ S ₅ ) are preferable.

The method for synthesizing the sulfide-based solid electrolyte is not particularly limited and may be appropriately selected depending on the intended purpose. For example, by mixing the starting materials and then firing at a temperature of 200 ° C. or higher and 500 ° C. or lower. can get. After that, after firing at a temperature of 600 ° C. or higher and 1,000 ° C. or lower, the sample may be rapidly cooled (quenched or quenched) using water, ice water, or the like, or slowly cooled to room temperature. .. Further, it may be fired at a temperature of 200 ° C. or higher and 500 ° C. or lower. This synthesis method, the lattice constant of the present invention is large and is required to tetragonal PS ₄ with Na defects.

As used herein, the term "conductivity" means the ionic conductivity of Na ions. In the following, the "conductivity of Na ions" may be simply referred to as "ion conductivity" or "conductivity".
The solid electrolyte of the present invention may contain other solid electrolytes in addition to the sulfide-based solid electrolyte, as long as the high conductivity caused by the sulfide-based solid electrolyte is not inhibited.
The ionic conductivity of the sulfide-based solid electrolyte can be measured by, for example, the AC impedance method.
The sulfide-based solid electrolyte preferably has a Na ion conductivity at 25 ° C. of 1.5 × 10 ^-4 S / cm or more, and more preferably 2.0 × 10 ^-4 S / cm or more.
The conductivity can be measured using, for example, an impedance gain phase analyzer (solartron 1260) manufactured by Solartron.

The sulfide-based solid electrolyte of the present invention can be used for various purposes because it can obtain high conductivity at the same level as that of the solid electrolyte for lithium secondary batteries. However, the solid electrolyte of the sodium battery described below can be used. Is particularly suitable as.

(Sodium battery)
The sodium battery of the present invention has a positive electrode, a negative electrode, and a sulfide-based solid electrolyte of the present invention, and further has other members as required.

The positive electrode is not particularly limited and may be appropriately selected depending on the intended purpose. For example, metals such as sulfur and titanium sulfide, sodium cobaltate, sodium manganate, sodium metallate and the like can be selected. Can be mentioned.
The negative electrode is not particularly limited and may be appropriately selected depending on the intended purpose. For example, an electrode containing an electrode active material selected from sodium metals, sodium alloys, and materials that can be doped or dedoped with sodium ions. Can be mentioned.

The sodium battery of the present invention may be a primary battery or a secondary battery, but a secondary battery is preferable.
Examples of the shape of the sodium battery include a coin type, a cylindrical type, a square type, and the like according to the shape of the container.
Examples of the other member include a separator, an outer can, an electrode take-out line, and the like.

Examples of the present invention will be described below, but the present invention is not limited to these examples.

(Example 1)
-Preparation of sulfide-based solid electrolyte 1-
In a glove box with an argon atmosphere, weigh 22.079 g of sodium sulfide (Na ₂ S) and 1.921 g of diphosphorus pentasulfide (P ₂ S ₅ ) as starting materials, mix them in an agate mortar for 10 minutes, and then vibrate. The starting materials were mixed using a mill.
As the vibration mill, TI-100 manufactured by CMT was used, and a pulverizing medium made of alumina was put therein together with the starting material, and the treatment was carried out at a rotation speed of 1,440 rpm for 30 minutes. As the pulverizing medium, a rod-shaped medium having a diameter of 5.3 cm × 5.5 cm was used.
Then, 0.1 g of the treated sample was prepared into pellets having a diameter of 1 mm at a display pressure of 10 MPa using a uniaxial press machine (manufactured by Riken Seiki Co., Ltd., P-6), and a carbon-coated quartz tube (with quartz) was prepared in advance. It was placed in a carbon coating to prevent a reaction) and vacuum-sealed at about 30 Pa.
The vacuum-sealed sample was heated to 450 ° C. over 3 hours in an electric furnace, maintained at 450 ° C. for 8 hours, and then slowly cooled to room temperature (25 ° C.).
Further, the temperature was raised to 700 ° C. over 3 hours in an electric furnace, maintained at 700 ° C. for 8 hours, and then the quartz tube containing the sample was placed in ice water to quench and hold for 3 hours.
Then, the temperature was raised again to 450 ° C. over 3 hours in an electric furnace, maintained at 450 ° C. for 8 hours, and then slowly cooled to room temperature (25 ° C.). From the above, [sulfide-based solid electrolyte 1] was obtained.
The composition of the obtained [sulfide-based solid electrolyte 1] was Na _{3-5 x} P _1-x S ₄ (x = 0.01) in terms of the amount charged.

(Example 2)
-Preparation of sulfide-based solid electrolyte 2-
[Sulfide-based solid electrolyte 2] was prepared in the same manner as in Example 1 except that the blending amount of the starting material was changed as follows in Example 1.
-Sodium sulfide (Na ₂ S): 2.052 g
-Phosphorus pentasulfide (P ₂ S ₅ ): 1.948 g
The composition of the obtained [sulfide-based solid electrolyte 2] was Na _{3-5 x} P _1-x S ₄ (x = 0.00) in terms of the amount charged.

(Example 3)
-Preparation of sulfide-based solid electrolyte 3-
[Sulfide-based solid electrolyte 3] was prepared in the same manner as in Example 1 except that the blending amount of the starting material was changed as follows in Example 1.
-Sodium sulfide (Na ₂ S): 2.025 g
-Phosphorus pentasulfide (P ₂ S ₅ ): 1.975 g
The composition of the obtained [sulfide-based solid electrolyte 3] was Na _{3-5 x} P _1-x S ₄ (x = −0.01) in terms of the amount charged.

(Example 4)
-Preparation of sulfide-based solid electrolyte 4-
[Sulfide-based solid electrolyte 4] was prepared in the same manner as in Example 1 except that the blending amount of the starting material was changed as follows in Example 1.
-Sodium sulfide (Na ₂ S): 1.998 g
-Phosphorus pentasulfide (P ₂ S ₅ ): 2.002 g
The composition of the obtained [sulfide-based solid electrolyte 4] was Na _{3-5 x} P _1-x S ₄ (x = −0.02) in terms of the amount charged.

(Comparative Example 1)
-Preparation of comparative sulfide-based solid electrolyte 1-
Weigh 22.025 g of sodium sulfide (Na ₂ S) and 1.975 g of diphosphorus pentasulfide (P ₂ S ₅ ) as starting materials in a glove box with an argon atmosphere, mix them in an agate mortar for 10 minutes, and then vibrate. The starting materials were mixed using a mill.
A TI-100 manufactured by CMT was used as the vibration mill, and a pulverizing medium made of alumina was put therein together with the sample, and the treatment was carried out at a rotation speed of 1,440 rpm for 30 minutes. As the pulverizing medium, a rod-shaped medium having a diameter of 5.3 cm × 5.5 cm was used.
Then, 0.1 g of the treated sample was prepared into pellets having a diameter of 1 mm at a display pressure of 10 MPa using a uniaxial press machine (manufactured by Riken Seiki Co., Ltd., P-6), and a carbon-coated quartz tube (with quartz) was prepared in advance. It was placed in a carbon coating to prevent a reaction) and vacuum-sealed at about 30 Pa.
The vacuum-sealed sample was heated to 450 ° C. over 3 hours in an electric furnace, maintained at 450 ° C. for 8 hours, and then slowly cooled to room temperature.
Further, the temperature was further raised to 700 ° C. over 3 hours in an electric furnace, maintained at 700 ° C. for 8 hours, and then the quartz tube containing the sample was placed in ice water to quench and hold for 3 hours. From the above, [Comparative Sulfide-based Solid Electrolyte 1] (Tetragonal Na ₃ PS ₄ ) was prepared.
The composition of the obtained [Comparative Sulfide-based Solid Electrolyte 1] was Na _{3-5 x} P _1-x S ₄ (x = −0.01) in terms of the amount charged.

(Comparative Example 2)
-Preparation of comparative sulfide-based solid electrolyte 2-
Weigh 22.105 g of sodium sulfide (Na ₂ S) and 1.895 g of diphosphorus pentasulfide (P ₂ S ₅ ) as starting materials in a glove box with an argon atmosphere, mix them in an agate mortar for 10 minutes, and then add more. The starting materials were mixed using a vibration mill.
A TI-100 manufactured by CMT was used as the vibration mill, and a pulverizing medium made of alumina was put therein together with the sample, and the treatment was carried out at a rotation speed of 1,440 rpm for 30 minutes. As the pulverizing medium, a rod-shaped medium having a diameter of 5.3 cm × 5.5 cm was used.
Then, 0.1 g of the treated sample was prepared into pellets having a diameter of 1 mm at a display pressure of 10 MPa using a uniaxial press machine (manufactured by Riken Seiki Co., Ltd., P-6), and a carbon-coated quartz tube (with quartz) was prepared in advance. It was placed in a carbon coating to prevent a reaction) and vacuum-sealed at about 30 Pa.
The vacuum-sealed sample was heated to 450 ° C. over 3 hours in an electric furnace, maintained at 450 ° C. for 8 hours, and then slowly cooled to room temperature (25 ° C.).
Further, the temperature was further raised to 700 ° C. over 3 hours in an electric furnace, maintained at 700 ° C. for 8 hours, and then the quartz tube containing the sample was placed in ice water to quench and hold for 3 hours. From the above, [Comparative Sulfide-based Solid Electrolyte 2] (cubic Na ₃ PS ₄ ) was prepared.
The composition of the obtained [Comparative Sulfide-based Solid Electrolyte 2] was Na _{3-5 x} P _1-x S ₄ (x = 0.02) in terms of the amount charged.

Next, the crystal structure, lattice constant a, lattice constant c, Na occupancy (deficiency), and conductivity of each of the produced sulfide-based solid electrolytes were measured as follows. The results are shown in Tables 1 and 2.

<Crystal structure>
For the sulfide-based solid electrolytes obtained in Examples 1 to 4 and Comparative Examples 1 and 2, powder X-ray diffraction measuring devices (Smart-Lab, Rigaku) using Cu-Kα rays having an X-ray wavelength of 1.5418 angstroms (Å). The X-ray diffraction pattern shown in FIG. 1 can be obtained, and the crystal structure can be identified from this diffraction pattern.

<Lattice constant a, lattice constant c, and Na occupancy (deficiency)>
For the sulfide-based solid electrolytes obtained in Examples 1 to 4 and Comparative Examples 1 and 2, the Rietveld method was used from the X-ray diffraction pattern shown in FIG. 1, and Z-Rietveld was used for the analysis program, and the lattice constant was used. The occupancy rate (deficiency) of a, the lattice constant c, and Na was determined.
Specifically, by applying the diffraction pattern calculated from the assumed structural model to the X-ray diffraction pattern obtained by the measurement and performing curve fitting, the occupancy rates of the lattice constant a, the lattice constant c, and Na ( Deficiency) was calculated.
FIG. 2 is a diagram showing the relationship between the lattice constant a obtained by Rietveld analysis from the X-ray diffraction diagram of FIG. 1 and the raw material charging composition for Examples 1 to 4.
FIG. 3 is a diagram showing the relationship between the lattice constant c obtained by Rietveld analysis from the X-ray diffraction diagram of FIG. 1 and the raw material charging composition for Examples 1 to 4.
FIG. 4 is a diagram showing the relationship between the conductivity and the raw material charging composition for Examples 1 to 4.
FIG. 6 is a diagram showing the crystal structure of the tetragonal Na ₃ PS ₄ without Na defects of Comparative Example 2.
FIG. 7 is a diagram showing the crystal structure of tetragonal Na ₃ PS ₄ having Na defects in Examples 1 to 4 and Comparative Example 1.

<Conductivity>
The sulfide-based solid electrolytes obtained in Examples 1 to 4 and Comparative Examples 1 and 2 were measured for Na ion conductivity at 25 ° C. by an AC impedance method.
First, in a glove box with an argon atmosphere, about 10 mg of gold powder (Niraco, dendritic, particle size of about 10 μm) was placed on both sides of the sample pellet and uniformly dispersed on the pellet surface, and the display pressure was 30 MPa ( Molding was performed at a molding pressure of about 560 MPa). The resulting pellets were then placed in a closed electrochemical cell capable of maintaining an argon atmosphere.
For the measurement, an impedance gain phase analyzer (solartron1260) manufactured by Solartron was used as a frequency response analyzer FRA (Freequency Response Analyzer), and a small environmental tester (Espec corp, SU-241) was used as a constant temperature device.
The measurement was started from a high frequency region under the conditions of an AC voltage of 10 mV to 1,000 mV, a frequency range of 1 Hz to 10 MHz, an integration time of 0.2 seconds, and a temperature of 25 ° C., and the conductivity was determined.

From the results in Table 1, it was found that in Examples 1 to 4, the lattice constant a was 6.948Å to 6.970Å and the lattice constant c was 7.087Å to 7.096Å, and the conductivity was high.
The conductivity of Example 2, which shows the highest conductivity among Examples 1 to 4, is 3.393 × 10 ^-3 [S / cm], which is 30 times to 2 times that of Comparative Examples 1 and 2. The conductivity was 000 times higher.
Furthermore, from the results in Table 2, the occupancy rate of Na1 and Na2 of ordinary tetragonal Na ₃ PS ₄ is 100% or less. That is, it was found that the conductivity is increased by having defects in Na1 and Na2.

(Example 5)
<Manufacturing sodium batteries>
-Preparation of electrodes-
TiS ₂ powder (manufactured by Wako Pure Chemical Industries, Ltd.) as an electrode active material, acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive agent, and polyvinylidene fluoride (PVdF, manufactured by Kureha Co., Ltd., # 1300) as an electrode forming agent. Weighed so as to have a composition of electrode active material: conductive agent: electrode forming agent = 8: 1: 1 (mass ratio).
First, the electrode-forming agent was added to a Menou dairy pot, and an appropriate amount of N-methyl-2-pyrrolidone (NMP, manufactured by Tokyo Kasei Kogyo Co., Ltd.) as a solvent was added thereto and sufficiently mixed to dissolve the electrode-forming agent. After confirming that, the electrode active material and the conductive agent were further added and sufficiently mixed to obtain an electrode mixture paste.
Next, the obtained electrode mixture paste was applied to a copper foil to a thickness of 100 μm using an applicator, placed in a vacuum dryer, and sufficiently dried while removing the solvent to obtain an electrode sheet. Obtained.
The obtained electrode sheet was sufficiently crimped with a roll press and then punched to a diameter of 1.0 cm with an electrode punching machine to obtain a positive electrode for a sodium battery.

-Preparation of solid electrolyte-
Using the sulfide-based solid electrolyte 2 prepared in Example 2, a disk-shaped solid electrolyte A having a diameter of 1.0 cm and a thickness of 0.70 mm was prepared.

-Manufacturing sodium secondary batteries-
Place the active material surface of the electrode as the positive electrode facing up in the recess of the lower part of the coin cell (manufactured by Hosen Co., Ltd.), and place the solid electrolyte A and sodium metal (manufactured by Kanto Chemical Co., Ltd.) as the negative electrode. Combined to produce a sodium secondary battery. The batteries were assembled in a glove box with an argon atmosphere.

<Evaluation of sodium secondary battery>
As a charging / discharging condition of the sodium secondary battery, constant current charging was performed at 1.0 mA / cm ² from the rest potential to 2.5 V. The discharge was a constant current discharge at 1.0 mA / cm ² and cut off at a voltage of 1.3 V. This charge / discharge was repeated for 50 cycles, and the transition of the discharge capacity is shown in FIG. The discharge capacity was measured using TOSCAT3100 (manufactured by Toyo System Co., Ltd.).
From the results shown in FIG. 5, since the discharge capacity remained stable, it was clear that the sulfide-based solid electrolyte used in the present invention could be used as a sodium secondary battery without any problem.

Aspects of the present invention are, for example, as follows.
<1> A sulfide-based solid electrolyte containing Na element, P element, and S element and having a composition of Na ₃ PS ₄ .
The crystal structure of Na ₃ PS ₄ is tetragonal.
The lattice constant a of Na ₃ PS ₄ is 6.948Å or more and 6.970Å or less.
It is a sulfide-based solid electrolyte characterized by having a lattice constant c of Na ₃ PS ₄ of 7.087 Å or more and 7.096 Å or less.
<2> The tetragonal Na ₃ PS ₄ Na of the sulfide-based solid electrolyte is the sulfide-based solid electrolyte according to <1>, which has a defect.
<3> In powder X-ray diffraction measurement using Cu-Kα rays with an X-ray wavelength of 1.5418Å, 17.8 °, 18.0 °, 31.0 °, 31.3 °, 36.1 °, and 36. The sulfide-based solid electrolyte according to any one of <1> to <2>, which has a peak near a diffraction angle (2θ) of 5 °.
<4> The sulfide-based solid according to any one of <1> to <3> above, wherein the Na ion conductivity at 25 ° C. measured by the AC impedance method is 1.5 × 10 ^-4 S / cm or more. It is an electrolyte.
<5> A sodium battery characterized by having a positive electrode, a negative electrode, and the sulfide-based solid electrolyte according to any one of <1> to <4>.
<6> The sodium battery according to <5>, which is a sodium secondary battery.

According to the sulfide-based solid electrolyte according to any one of <1> to <4> and the sodium battery according to any one of <5> to <6>, the above-mentioned problems in the prior art can be solved. The object of the present invention can be achieved.

International Publication No. 2013/133020 Pamphlet

A. Hayashi et. al, Superior Glass-Ceramic Electrolytes for Room-Temperature Rechargeable Batteries, Nature Communications, 3 (2012) 856: 1-5 Noriaki Kamaya, et. al, Aluminum superionic conductor, Nature Materials, 10, 682-686 (2011)

Claims (6)

A sulfide-based solid electrolyte containing Na element, P element, and S element and having a composition of Na ₃ PS ₄ .
The crystal structure of Na ₃ PS ₄ is tetragonal.
The lattice constant a of Na ₃ PS ₄ is 6.948Å or more and 6.970Å or less.
A sulfide-based solid electrolyte characterized in that the lattice constant c of Na ₃ PS ₄ is 7.087 Å or more and 7.096 Å or less. The sulfide-based solid electrolyte according to claim 1, wherein the tetragonal Na ₃ PS ₄ Na of the sulfide-based solid electrolyte has a defect. In powder X-ray diffraction measurements with Cu-Kα rays with an X-ray wavelength of 1.5418Å, 17.8 °, 18.0 °, 31.0 °, 31.3 °, 36.1 °, and 36.5 ° The sulfide-based solid electrolyte according to any one of claims 1 to 2, which has a peak near the diffraction angle (2θ). The sulfide-based solid electrolyte according to any one of claims 1 to 3, wherein the Na ion conductivity at 25 ° C. measured by an AC impedance method is 1.5 × 10 ^-4 S / cm or more. A sodium battery comprising a positive electrode, a negative electrode, and a sulfide-based solid electrolyte according to any one of claims 1 to 4. The sodium battery according to claim 5, which is a sodium secondary battery.