Solid electrolyte and secondary battery

Abstract

The present invention improves ion conductivity while improving water resistance. Provided is a solid electrolyte having a composition formula represented by LixMXy, wherein M is one or more elements selected from group 14 elements, X includes S, and the compositional ratio of Li, M, and X is x:1:y. The crystal structure of the solid electrolyte is hexagonal, and the volume of a hexagonal unit cell is greater than 0.0870 nm3 but less than 0.0880 nm3.

Description

Solid electrolytes and secondary batteries

[0001] The present disclosure relates to a solid electrolyte and a secondary battery.

[0002] In Non-Patent Document 1, the composition formula is Li ４ SnS ４ A solid electrolyte is described in which

[0003] Inorganic Chemistry, 2018, Vol 57, No. 16, p. 9925-9930

[0004] The solid electrolyte disclosed in Non-Patent Document 1 has excellent water resistance, but there is a possibility that the ionic conductivity may decrease.

[0005] The present disclosure has been made in view of the above, and aims to improve water resistance and ion conductivity.

[0006] The solid electrolyte according to one embodiment has a composition formula of Li ｘ MX ｙ M is one or more elements selected from the group 14 elements, X includes S, Li, M, and X have a composition ratio of x:1:y, the crystal structure is hexagonal, and the volume of the unit cell of the hexagonal crystal is 0.0870 nm ３ Larger than 0.0880 nm ３ Smaller than.

[0007] A secondary battery according to one embodiment includes a positive electrode, a negative electrode having a negative electrode active material layer containing a negative electrode active material, and an electrolyte layer containing the solid electrolyte.

[0008] A secondary battery according to another embodiment includes a positive electrode, a negative electrode including a particulate negative electrode active material and the solid electrolyte, and an electrolyte layer.

[0009] According to the present invention, it is possible to improve water resistance and ionic conductivity at the same time.

[0010] FIG. 1 is a schematic cross-sectional view showing an example of a battery according to a first embodiment. FIG. 2 is a schematic cross-sectional view showing an example of a battery according to a second embodiment. FIG. 3 is a schematic cross-sectional view showing an example of a battery according to a first modified example of the second embodiment. FIG. 4 is a schematic cross-sectional view showing an example of a battery according to a second modified example of the second embodiment. FIG. 5 is a schematic cross-sectional view showing an example of a battery according to a third embodiment. FIG. 6 is a diagram showing X-ray diffraction charts according to Example 1 and Comparative Example 1.

[0011] Hereinafter, embodiments of the present disclosure will be described. Note that the present disclosure is not limited to these embodiments. Furthermore, in the present disclosure, numerical values ​​include ranges that are rounded off.

[0012] First Embodiment FIG. 1 is a schematic cross-sectional view showing an example of a battery according to a first embodiment. The battery 1 in the first embodiment is an all-solid-state battery in which the electrolyte is solid, and is a lithium-ion secondary battery. As shown in FIG. 1, the battery 1 includes a protective layer 10, a positive electrode 20, a negative electrode 30, and an electrolyte layer 40. In the example of FIG. 1, the battery 1 has a structure in which the sheet-like positive electrode 20, the negative electrode 30, and the electrolyte layer 40 are stacked.

[0013] In the drawings showing this embodiment, the Z direction refers to the stacking direction of the positive electrode 20, the negative electrode 30, and the electrolyte layer 40, the X direction refers to a direction perpendicular to the Z direction and parallel to the cross section of FIG. 1, and the Y direction refers to a direction perpendicular to the X direction and the Z direction. In addition, in describing this embodiment, one of the X directions may be referred to as the +X direction and the other as the −X direction. Similarly, one of the Z directions may be referred to as the +Z direction and the other as the −Z direction.

[0014] The protective layer 10 is a layer provided to physically and chemically protect the battery 1. In plan view in the Z direction, the protective layer 10 is provided so as to overlap the stack of the positive electrode 20, the negative electrode 30, and the electrolyte layer 40, and in the example of Fig. 1, the protective layer 10 is provided on both sides in the Z direction of the stack of the positive electrode 20, the negative electrode 30, and the electrolyte layer 40. The material of the protective layer 10 is not particularly limited as long as it is an insulator, and examples thereof include resin, glass, and ceramics.

[0015] The positive electrode 20 includes a positive electrode current collector layer 21 and a positive electrode active material layer 22 .

[0016] The positive electrode current collector layer 21 is a conductive layer. In the example of FIG. 1 , the end face of the positive electrode current collector layer 21 in the +X direction is exposed and can be connected to the outside. That is, the end face of the positive electrode current collector layer 21 in the +X direction serves as the positive electrode of the battery 1. The material of the positive electrode current collector layer 21 is not particularly limited as long as it is conductive, and examples thereof include metal materials such as silver, palladium, gold, platinum, aluminum, copper, and nickel, and carbon materials.

[0017] The positive electrode active material layer 22 is a layer containing a positive electrode active material. The positive electrode active material layer 22 is laminated on the positive electrode current collector layer 21. The positive electrode active material is not particularly limited, and examples thereof include at least one selected from the group consisting of a lithium-containing phosphate compound having a Nasicon structure, a lithium-containing phosphate compound having an olivine structure, a lithium-containing layered oxide, and a lithium-containing oxide having a spinel structure. An example of a lithium-containing phosphate compound having a Nasicon structure is Li ３ V ２ (P.O. ４ ) ３ Examples of lithium-containing phosphate compounds having an olivine structure include Li ３ Fe ２ (P.O. ４ ) ３ , LiMnPO ４ An example of a lithium-containing layered oxide is LiCoO ２ , LiCo １／３ Ni １／３ Mn １／３ O ２ An example of a lithium-containing oxide having a spinel structure is LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ etc.

[0018] The material contained in the positive electrode active material layer 22 is not limited to the positive electrode active material, and may also contain a solid electrolyte or a sintering aid, which will be described later. The sintering aid is not particularly limited, and examples thereof include lithium oxide, sodium oxide, potassium oxide, boron oxide, silicon oxide, bismuth oxide, and phosphorus oxide.

[0019] The negative electrode 30 includes a negative electrode current collector layer 31 and a negative electrode active material layer 32 .

[0020] The negative electrode current collector layer 31 is a conductive layer. In the example of FIG. 1 , the negative electrode current collector layer 31 has an exposed end face in the −X direction that can be connected to the outside. That is, the −X direction end face of the negative electrode current collector layer 31 serves as the negative electrode of the battery 1. The material of the negative electrode current collector layer 31 is a conductive metal and contains at least one metal selected from the group consisting of copper, nickel, and iron. However, the material of the negative electrode current collector layer 31 is not limited to these, and may further contain, for example, at least one metal material, such as palladium, gold, platinum, or aluminum, and a carbon material. Furthermore, the negative electrode current collector layer 31 is not limited to being composed of a single layer, and may include multiple layers, such as stainless steel coated with nickel on the negative electrode active material layer 32 side.

[0021] The negative electrode active material layer 32 is a layer containing a negative electrode active material. In the example of Fig. 1, the negative electrode active material layer 32 is provided in the +Z direction of the negative electrode current collector layer 31. The negative electrode active material refers to a substance that can absorb and release carrier ions such as lithium ions through a charge / discharge reaction.

[0022] Examples of materials capable of absorbing and releasing lithium ions that can be used as negative electrode active materials include oxides containing at least one element selected from the group consisting of titanium (Ti), silicon (Si), tin (Sn), chromium (Cr), iron (Fe), niobium (Nb), and molybdenum (Mo), carbon materials such as graphite, graphite-lithium compounds, lithium alloys, lithium-containing phosphate compounds having a Nasicon structure, lithium-containing phosphate compounds having an olivine structure, and lithium-containing oxides having a spinel structure. An example of a lithium alloy is a Li-Al alloy. An example of a lithium-containing phosphate compound having a Nasicon structure is Li. ３ V ２ (P.O. ４ ) ３ , LiTi ２ (P.O. ４ ) ３ Examples of lithium-containing phosphate compounds having an olivine structure include Li ３ Fe ２ (P.O. ４ ) ３ , LiCuPO ４ Examples of lithium-containing oxides having a spinel structure include Li ４ Ti ５ O １２ etc.

[0023] The negative electrode active material may be a material capable of absorbing and releasing sodium ions through charge-discharge reactions. That is, the battery according to the present disclosure may be a sodium-ion battery. In this case, examples of materials capable of absorbing and releasing sodium ions that can be used as the negative electrode active material include at least one selected from the group consisting of sodium-containing phosphate compounds having a Nasicon structure, sodium-containing phosphate compounds having an olivine structure, and sodium-containing oxides having a spinel structure.

[0024] In the first embodiment, the anode active material layer 32 is a film made of anode active material. That is, the anode active material layer 32 is a layer in which the anode active material is continuous from the surface on the −Z direction side (the surface on the anode current collector layer 31 side) to the surface on the +Z direction side (the surface on the electrolyte layer 40 side). In other words, the anode active material layer 32 does not substantially contain components of the electrolyte layer 40 (e.g., solid electrolyte). Examples of the continuous body include a pressure-molded body or sintered compact of only the anode active material, or a film formed by plating, sputtering, vapor deposition, or the like, but may also be a metal foil, a wafer, or the like.

[0025] The electrolyte layer 40 is a layer that does not substantially contain a positive electrode active material or a negative electrode active material, and is a layer provided between the positive electrode 20 and the negative electrode 30. The electrolyte layer 40 contains a solid electrolyte. In the first embodiment, the electrolyte layer 40 is a laminate having a first electrolyte layer 41 and a second electrolyte layer 42.

[0026] The first electrolyte layer 41 is a layer on the negative electrode 30 side of the electrolyte layer 40. In the first embodiment, the first electrolyte layer 41 is made of a first solid electrolyte. That is, the surface of the negative electrode active material of the negative electrode active material layer 32 on the +Z direction side (electrolyte layer 40 side) is coated with the first solid electrolyte. This allows the surface of the negative electrode active material to be covered with the first solid electrolyte, which has high ion conductivity, thereby improving charge / discharge characteristics. Here, whether the surface of the negative electrode active material on the electrolyte layer 40 side is coated with the first solid electrolyte is determined by the following method. First, a cross section of the battery 1 along the stacking direction is observed with a scanning electron microscope (SEM) to identify the interface between the negative electrode active material layer 32 and the electrolyte layer 40. Next, an arbitrary point near the interface on the electrolyte layer 40 side is measured with an energy dispersive X-ray fluorescence spectrometer (EDX). In this case, if a spectrum of the Group 14 element M derived from the first solid electrolyte is observed in the spectrum observed by EDX, it can be determined that the surface of the negative electrode active material on the electrolyte layer 40 side is covered with the first solid electrolyte. Here, the vicinity of the interface on the electrolyte layer 40 side refers to a region up to a surface 300 nm away from the interface in the stacking direction.

[0027] The first solid electrolyte is a solid electrolyte according to the present invention, and has the composition formula Li ｘ MX ｙ That is, the composition ratio of Li, M, and X in the first solid electrolyte is x:1:y. Here, y preferably satisfies 3.0≦y≦5.0. By setting y in this range, the crystal structure of the first solid electrolyte can be formed by Li ４ SnS ４ Since the structure can be the same as that of the first solid electrolyte, water resistance can be improved. Furthermore, x preferably satisfies 3.90<x<4.00 or 4.00<x≦4.10. By slightly changing the volume of the unit cell of the first solid electrolyte from the state where x = 4.00, i.e., the state based on the stoichiometric ratio, the volume of the hexagonal unit cell of the first solid electrolyte can be increased, thereby improving ionic conductivity while maintaining water resistance. Here, x and y can be obtained by obtaining the molar ratio of X (e.g., sulfur (S)) to M from the spectral intensity measured by windowless energy dispersive X-ray spectroscopy (Window-Less EDX), inductively coupled plasma optical emission spectroscopy (ICP-OES), or glow discharge optical emission spectrometry (GD-OES) using the electrolyte layer or negative electrode removed from the battery as a sample. When measuring x and y by ICP-OES, the solid electrolyte is extracted with water or methanol from the electrolyte layer or negative electrode taken out of the battery and used as a sample for ICP-OES measurement.

[0028] The composition formula of the first solid electrolyte is Li ｘ MX ｙIn the formula, M is one or more elements selected from the group 14 elements. In the present disclosure, the group 14 elements refer to a group of elements consisting of carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb). This improves water resistance while also improving ionic conductivity. M preferably contains Sn. This improves water resistance. More preferably, M further contains Si. This increases the volume V of the unit cell, thereby further improving ionic conductivity. In addition, the composition formula of the first solid electrolyte Li ｘ MX ｙ In the formula (I), X contains sulfur (S). This can improve ionic conductivity. Note that X is not limited to sulfur (S) only, and may contain, for example, oxygen (O).

[0029] The crystal structure of the first solid electrolyte is hexagonal. This can improve the discharge capacity of the battery 1. The crystal structure of the first solid electrolyte can be measured by X-ray diffraction (XRD) using the electrolyte layer or the negative electrode removed from the battery as a sample. Here, in the XRD measurement of the first solid electrolyte, the crystal group P6 ３ When a peak attributable to / mmc is observed, it can be said that the crystal structure of the first solid electrolyte is hexagonal. The crystal structure of the first solid electrolyte may be measured by a transmission electron microscope (TEM) instead of XRD. In this case, in the electron diffraction image obtained by the TEM, the crystal group P6 ３ When a peak attributed to / mmc is observed, it can be said that the crystal structure of the first solid electrolyte is hexagonal.

[0030] The volume of the hexagonal unit cell of the first solid electrolyte is 0.0870 nm ３ Larger than 0.08709 nm ３ This allows lithium ions to pass through the first solid electrolyte well, improving ionic conductivity. The volume of the hexagonal unit cell of the first solid electrolyte is 0.0880 nm or more. ３ Smaller, 0.08795 nm ３ Preferably, it is 0.08772 nm or less.３ This can be said to reduce the repulsive force between lithium ions in the first solid electrolyte, thereby improving ion conductivity.

[0031] The volume of the hexagonal unit cell of the first solid electrolyte can be measured by X-ray diffraction (XRD) using the electrolyte layer or negative electrode removed from the battery as a sample. More specifically, the volume V of the hexagonal unit cell can be calculated by performing Rietveld analysis on the diffraction chart obtained by XRD measurement of the first solid electrolyte to derive the lattice constants a and c. The crystal structure of the first solid electrolyte may be measured using a transmission electron microscope (TEM) instead of XRD. In this case, the volume V of the hexagonal unit cell can be calculated by measuring the lattice constants a and c from the electron diffraction image obtained by the TEM.

[0032] The thickness of the first electrolyte layer 41 is preferably 100 nm or more, and more preferably 200 nm or more, so that the surface of the negative electrode active material is sufficiently covered with the first solid electrolyte having high ion conductivity, thereby improving the charge / discharge characteristics.

[0033] The second electrolyte layer 42 is a layer located on the +Z direction side of the first electrolyte layer 41 and is made of a second solid electrolyte different from the first solid electrolyte. The second solid electrolyte is not particularly limited as long as it is a material through which ions can move. The second solid electrolyte is, for example, Li ６ P.S. ５ Cl, Li ３ P.S. ４ , Li ４ SnS ４ and the like.

[0034] The side reinforcing portions 60 are provided to prevent short circuits in the battery 1. In the example of Fig. 1, the side reinforcing portions 60 are provided on the X-direction and Y-direction end surfaces of the positive electrode 20, the negative electrode 30, and the electrolyte layer 40. The material of the side reinforcing portions 60 is not particularly limited as long as it is an insulator, and examples thereof include resin, glass, and ceramics.

[0035] The negative electrode and battery according to the first embodiment are not limited to those described above. For example, if the negative electrode active material layer contains a conductive material, the negative electrode may not have a negative electrode current collector layer. In this case, the negative electrode active material layer serves as the negative electrode of the battery and can be connected to the outside.

[0036] The battery according to the first embodiment may also be a battery having an exterior body (case). That is, the battery according to the first embodiment may be a battery in which a laminate including the positive electrode 20, the negative electrode 30, and the electrolyte layer 40 is housed in an exterior body made of metal, ceramics, or the like.

[0037] Furthermore, the second electrolyte layer 42 is not an essential component, and the electrolyte layer 40 does not necessarily have to have the second electrolyte layer 42. That is, the surface of the first electrolyte layer 41 opposite to the negative electrode may be in contact with the positive electrode 20.

[0038] As described above, the solid electrolyte (first solid electrolyte) according to this embodiment has the composition formula Li ｘ MX ｙ where M is one or more elements selected from the Group 14 elements, X includes S, and the composition ratio of Li, M, and X is x:1:y. The crystal structure is hexagonal, and the volume of the hexagonal unit cell is 0.0870 nm ３ Larger than 0.0880 nm ３ This makes it possible to improve water resistance and ionic conductivity.

[0039] In a preferred embodiment, the volume of the hexagonal unit cell is 0.08709 nm ３ 0.08795nm or more ３ This makes it possible to improve water resistance and further improve ion conductivity.

[0040] In a preferred embodiment, M contains Sn, which can improve water resistance.

[0041] In a more desirable embodiment, M further contains Si, which can further improve ionic conductivity.

[0042] In a desirable embodiment, x satisfies 3.90<x<4.00 or 4.00<x≦4.10, and y satisfies 3.0≦y≦5.0, thereby improving ionic conductivity while maintaining water resistance.

[0043] As described above, the secondary battery according to this embodiment includes the positive electrode 20, the negative electrode 30 having the negative electrode active material layer 32 containing the negative electrode active material, and the electrolyte layer 40 containing the solid electrolyte (first solid electrolyte) according to this embodiment. This improves water resistance and ionic conductivity.

[0044] The negative electrode active material layer 32 may also be a film made of a negative electrode active material. In this case, the water resistance and ion conductivity can be improved.

[0045] In a preferred embodiment, the surface of the negative electrode active material layer 32 facing the electrolyte layer 40 is coated with a solid electrolyte. This allows the surface of the negative electrode active material to be covered with the first solid electrolyte having high ion conductivity, thereby improving the charge / discharge characteristics.

[0046] Second Embodiment Fig. 2 is a schematic cross-sectional view showing an example of a battery according to a second embodiment. As shown in Fig. 2, the negative electrode 30A of the battery 1A according to the second embodiment differs from the first embodiment in that the negative electrode active material is particulate and the negative electrode active material layer 32A contains a first solid electrolyte. The second embodiment will be described below with reference to Fig. 2.

[0047] In the present disclosure, when the negative electrode active material is in a particulate form, a plane 30a that passes through the point on the surface of the negative electrode active material particle that is farthest from the surface of the negative electrode current collector layer in the Z direction and that is parallel to the surface of the negative electrode current collector layer is defined as the interface between the negative electrode 30 and the electrolyte layer 40. In the example of FIG. 2 , the interface between the negative electrode 30 and the electrolyte layer 40 corresponds to plane 30a that passes through the point on the surface of the negative electrode active material particle that is furthest in the +Z direction and is parallel to the X and Y directions. That is, in the example of FIG. 2 , the side of plane 30a that is closer to the negative electrode current collector layer 31 than plane 30a is the negative electrode active material layer 32A, and the side of plane 30a that is closer to the positive electrode 20 than plane 30a is the electrolyte layer 40.

[0048] In the second embodiment, the anode active material layer 32A includes a particulate anode active material 32a and a first solid electrolyte 32b. Here, "particulate" refers to the anode active material layer not being a continuous film. That is, in the second embodiment, the anode active material particles are intermittently arranged from the surface on the -Z direction side (the surface on the anode current collector layer 31 side) to the surface on the +Z direction side (the surface on the electrolyte layer 40 side).

[0049] In the second embodiment, the surface of the particulate anode active material 32a is coated with a first solid electrolyte 32b. In the example shown in FIG. 2, the first solid electrolyte 32b surrounds the particles of the anode active material 32a. In other words, in the anode active material layer 32A according to the second embodiment, the particles of the anode active material 32a are dispersed in the first solid electrolyte 32b. This allows the surface of the anode active material to be coated with the first solid electrolyte, which has high ion conductivity, thereby improving charge / discharge characteristics. Whether the surface of the particulate anode active material 32a is coated with the first solid electrolyte 32b is determined by the following method. First, a cross section of the battery 1 along the stacking direction is observed with a scanning electron microscope (SEM) to identify the interface between the particulate anode active material 32a and the first solid electrolyte 32b. Next, EDX measurement is performed at an arbitrary point near the interface on the first solid electrolyte 32b side. In this case, if a spectrum of the Group 14 element M derived from the first solid electrolyte is observed in the spectrum observed by EDX, it can be determined that the surface of the particulate negative electrode active material 32 a is covered with the first solid electrolyte 32 b. Here, the vicinity of the interface refers to a region from the interface to a surface 300 nm away in the normal direction to the surface of the particulate negative electrode active material 32 a.

[0050] The battery according to the second embodiment is not limited to the one described above. FIG. 3 is a schematic cross-sectional view showing an example of a battery according to a first modification of the second embodiment. FIG. 4 is a schematic cross-sectional view showing an example of a battery according to a second modification of the second embodiment. The battery according to the second embodiment may be the batteries 1B and 1C shown in FIGS. 3 and 4. In the first modification, the electrolyte layer 40A is a single layer made of the first solid electrolyte. In the second modification, the electrolyte layer 40B is a single layer made of the second solid electrolyte. Even in this case, the ionic conductivity can be improved.

[0051] As described above, the secondary battery according to the second embodiment includes the positive electrode 20, the negative electrode 30 including the particulate negative electrode active material 32 a and the solid electrolyte according to the first embodiment (first solid electrolyte 32 b), and the electrolyte layer 40. Even in this case, it is possible to improve water resistance and ionic conductivity.

[0052] In a preferred embodiment, at least a portion of the surface of the particles of the negative electrode active material 32 a is coated with a solid electrolyte (first solid electrolyte 32 b), which has high ion conductivity, thereby improving charge / discharge characteristics.

[0053] 5 is a schematic cross-sectional view showing an example of a battery according to the third embodiment. As shown in Fig. 5, in the anode 30B of the battery 1D according to the third embodiment, the anode active material is particulate, and the anode active material layer 32B includes a first solid electrolyte 32b and a second solid electrolyte 32c, which are different from those of the first embodiment.

[0054] In the third embodiment, the anode active material layer 32B includes particulate anode active material 32a, a first solid electrolyte 32b, and a second solid electrolyte 32c. In the example of FIG. 5 , the first solid electrolyte 32b coats the particles of the anode active material 32a, and the second solid electrolyte 32c surrounds the particles of the anode active material 32a coated with the first solid electrolyte 32b. In other words, in the anode active material layer 32A according to the third embodiment, particles of the anode active material 32a coated with the first solid electrolyte 32b are dispersed in the second solid electrolyte 32c. This allows the surface of the anode active material 32a to be coated with the first solid electrolyte 32b, thereby improving charge / discharge characteristics. Here, whether the surface of the particulate anode active material 32a is coated with the first solid electrolyte 32b is determined by the following method. First, a cross section of the battery 1 along the stacking direction is observed with a scanning electron microscope (SEM) to identify the interface between the particulate anode active material 32a and the first solid electrolyte 32b. Next, EDX measurement is performed at an arbitrary point near the interface on the first solid electrolyte 32b side. In this case, if a spectrum of the Group 14 element M derived from the first solid electrolyte is observed in the spectrum observed by EDX, it can be determined that the surface of the particulate anode active material 32a is coated with the first solid electrolyte 32b. Here, the "near the interface" refers to a region extending from the interface to a surface 300 nm away from the interface in the normal direction to the surface of the particulate anode active material 32a. It is sufficient that at least a portion of the surface of the particles of the anode active material 32a is coated with the first solid electrolyte; more preferably, the entire surface of the particles of the anode active material 32a is coated with the first solid electrolyte.

[0055] The thickness of the first solid electrolyte 32b covering the particulate negative electrode active material 32a is preferably 100 nm or more, and more preferably 200 nm or more. Here, the thickness of the first solid electrolyte 32b refers to the average length of the first solid electrolyte 32b in the direction normal to the surface of the particles of the negative electrode active material 32a. This allows the surface of the negative electrode active material 32a to be sufficiently covered with the first solid electrolyte 32b, which has high ion conductivity, thereby further improving charge / discharge characteristics.

[0056] In the example of FIG. 5, the electrolyte layer 40A is a single layer made of the second solid electrolyte, but this is merely an example and is not limited to this. For example, the electrolyte layer 40A may be a single layer made of the first solid electrolyte, or a laminate of the first solid electrolyte and the second solid electrolyte.

[0057] Examples of the present embodiment will be described below, but the present embodiment is not limited to the following examples.

[0058] Example 1 The first solid electrolyte according to Example 1 was prepared by the following method. ２ S powder, Sn powder, and S powder were weighed and mixed in a molar ratio of 2.05:1.00:1.95. Next, the total amount (g) of the raw material mixture was dissolved in pure water to a solution concentration of 15 wt %, and the mixture was stirred for 24 hours while being heated at 80°C. The resulting mixture was then vacuum dried at 150°C for 3 hours to obtain a powder of the first solid electrolyte according to Example 1. The composition of the first solid electrolyte according to Example 1 was Li ４．１０ SnS ４ It was.

[0059] <Crystal Structure Analysis> Crystal structure analysis was performed on the first solid electrolyte according to Example 1 by the following method. The obtained powder of the first solid electrolyte was filled into a quartz glass capillary with an inner diameter of 0.3 mm in an inert gas, and a sealed sample was obtained. XRD measurement was performed on the sample under the following conditions. XRD: BL5S2 (Aichi Synchrotron Light Center) Synchrotron radiation output: 15.5 keV The diffraction chart obtained by XRD was analyzed using XRD analysis software (SmartLabStudioII (SLS2), manufactured by Rigaku Co., Ltd.) to calculate the volume V of the unit cell. Specifically, cerium oxide (CeO ２ ) was used as an angle standard to perform angle correction, and then Rietveld analysis was performed by fitting using the Whole Powder Pattern Fitting (WPPF) method to derive the lattice constants a and c, and the volume V of the unit cell was calculated based on equation (1).

[0060] <Water Resistance Evaluation> The water resistance of the first solid electrolyte according to Example 1 was evaluated by the following method. 100 mg of the obtained first solid electrolyte and a hydrogen sulfide gas sensor were placed in a sealed container with a volume of 1 L and exposed to air with a dew point temperature of −30° C. for 1 hour. At this time, the hydrogen sulfide concentration in the sealed container was measured by reading the measurement value of the hydrogen sulfide sensor.

[0061] <Ionic Conductivity Measurement> The ionic conductivity of the first solid electrolyte according to Example 1 was measured by the following method. 100 mg of the obtained first solid electrolyte was pressed using a ceramic insulating cylinder with a diameter of 10 mm at room temperature and a pressure of 294 MPa to obtain a pellet. Thereafter, an electrochemical impedance spectrum was measured at an amplitude of 10 mV and a frequency of 106 Hz-10 Hz while a pressure of 98 MPa was applied to the pellet. The ionic conductivity of the first solid electrolyte according to Example 1 was calculated from the resistance value obtained from the spectrum and the thickness of the pellet.

[0062] Example 2 In Example 2, the raw material of the first solid electrolyte was Li ２ The first solid electrolyte was prepared and measured in the same manner as in Example 1, except that the first electrolyte was prepared by mixing S powder, Sn powder, and S powder in a molar ratio of 2.03:1.00:1.97. The composition of the first solid electrolyte according to Example 2 was Li ４．０６ SnS ４ It was.

[0063] Example 3 In Example 3, the raw material of the first solid electrolyte was Li ２ The first solid electrolyte was prepared and measured in the same manner as in Example 1, except that the first electrolyte was prepared by mixing S powder, Sn powder, and S powder in a molar ratio of 2.02:1.00:1.98. The composition of the first solid electrolyte according to Example 3 was Li ４．０４ SnS ４ It was.

[0064] Example 4 In Example 4, the raw material of the first solid electrolyte was Li ２The first solid electrolyte was prepared and measured in the same manner as in Example 1, except that the first electrolyte was prepared by mixing S powder, Sn powder, and S powder in a molar ratio of 2.01:1.00:1.99. The composition of the first solid electrolyte according to Example 4 was Li ４．０２ SnS ４ It was.

[0065] Example 5 In Example 5, the raw material of the first solid electrolyte was Li ２ The first solid electrolyte was prepared and measured in the same manner as in Example 1, except that the first electrolyte was prepared by mixing S powder, Sn powder, Si powder, and S powder in a molar ratio of 2.01:0.90:0.10:1.99. The composition of the first solid electrolyte according to Example 5 was Li ４．０２ Sn ０．９０ Si ０．１０ S ４ It was.

[0066] Example 6 In Example 6, the raw material of the first solid electrolyte was Li ２ The first solid electrolyte was prepared and measured in the same manner as in Example 1, except that the first electrolyte was prepared by mixing S powder, Sn powder, Si powder, and S powder in a molar ratio of 2.01:0.95:0.05:1.99. The composition of the first solid electrolyte according to Example 6 was Li ４．０２ Sn ０．９５ Si ０．０５ S ４ It was.

[0067] Example 7 In Example 7, the raw material of the first solid electrolyte was Li ２ The first solid electrolyte was prepared and measured in the same manner as in Example 1, except that the first electrolyte was prepared by mixing S powder, Sn powder, and S powder in a molar ratio of 1.96:1.00:2.04. The composition of the first solid electrolyte according to Example 7 was Li ３．９２ SnS ４ It was.

[0068] Comparative Example 1 In Comparative Example 1, the raw material of the first solid electrolyte was Li ２The first solid electrolyte was prepared and measured in the same manner as in Example 1, except that the first electrolyte was prepared by mixing S powder, Sn powder, and S powder in a molar ratio of 2.0:1.0:2.0. The composition of the first solid electrolyte according to Comparative Example 1 was Li ４．００ SnS ４．００ It was.

[0069] Comparative Example 2 In Comparative Example 2, the raw material of the first solid electrolyte was Li ２ The first solid electrolyte was prepared and measured in the same manner as in Example 1, except that the first electrolyte was prepared by mixing S powder, Sn powder, and S powder in a molar ratio of 1.95:1.0:2.05. The composition of the first solid electrolyte according to Comparative Example 2 was Li ３．９０ SnS ３．００ It was.

[0070] Comparative Example 3 In Comparative Example 3, Li was used as a raw material for the first solid electrolyte. ２ S powder and SnS ２ The powders were charged into a planetary ball mill (Fritsch, P-7) in a molar ratio of 2.0:1.0, and ball mill treatment was performed for 50 hours at a rotation speed of 510 rpm to produce a first solid electrolyte by mechanical milling reaction. Except for this, the first solid electrolyte was produced and measured in the same manner as in Example 1. Here, the inner container of the planetary ball mill used to produce the first solid electrolyte according to Comparative Example 3 was a zirconia container with a volume of 45 mL, and 90 g of zirconia balls with a diameter of 4 mm were charged into the inner container together with the raw materials and processed. The composition of the first solid electrolyte according to Comparative Example 3 was Li ４．００ SnS ４．００ It was.

[0071] Table 1 shows the compositions of the first solid electrolytes according to Examples 1 to 7 and Comparative Examples 1 to 3, as well as the measurement results.

[0072]

[0073] In the water resistance test, the hydrogen sulfide gas sensors of Examples 1 to 7 and Comparative Examples 1 to 3 continued to exhibit values ​​below the lower measurement limit of 0.1 ppm. This shows that the first solid electrolytes of Examples 1 to 7, like the first solid electrolytes of Comparative Examples 1 and 2, have sufficient water resistance.

[0074] In the crystal structure analysis, in Examples 1 to 7 and Comparative Examples 1 to 3, the crystal structure was hexagonal, and the volume of the unit cell was the value shown in Table 1. FIG. 6 is a diagram showing X-ray diffraction charts for Example 1 and Comparative Example 1. As shown in FIG. 6, the X-ray diffraction chart for Example 1 is shifted to the high-angle side compared to Comparative Example 1. This shows that the solid electrolyte for Example 1 has a larger unit cell volume than the solid electrolyte for Comparative Example 1. Note that the larger volume of the unit cell in Comparative Example 3 is thought to be due to the generation of amorphous portions in the solid electrolyte due to mechanical synthesis.

[0075] As shown in Table 1, the volume of the hexagonal unit cell is 0.0870 nm ３ Larger than 0.0880 nm ３ In smaller Examples 1 to 7, the volume of the hexagonal unit cell is 0.0870 nm ３ Comparative Example 1 and Comparative Example 2 ３ The ionic conductivity was improved compared to Comparative Examples 2 and 3. As a result, the volume of the hexagonal unit cell of the first solid electrolyte was 0.0870 nm ３ Larger than 0.0880 nm ３ It can be seen that a smaller size can improve ionic conductivity.

[0076] As shown in Table 1, the volume of the hexagonal unit cell is 0.08709 nm ３ 0.08795nm or more ３ In Examples 1 to 7, the volume of the hexagonal unit cell is 0.08709 nm ３ Smaller Comparative Example 1 and 0.08795 nm ３ The ionic conductivity was improved compared to Comparative Examples 2 and 3, which had a larger ionic conductivity. As a result, the volume of the hexagonal unit cell of the first solid electrolyte was 0.08709 nm ３ 0.08795nm or more ３ It can be seen that the ionic conductivity can be improved by satisfying the following.

[0077] As shown in Table 1, Examples 1 to 6 where x was 4.00<x≦4.10 and Example 7 where x was 3.90<x<4.00 had improved ionic conductivity compared to Comparative Examples 1 and 3 where x was 4.00 and Comparative Example 2 where x was ≦3.90. This shows that ionic conductivity can be improved by satisfying 3.90<x<4.00 or 4.00<x≦4.10.

[0078] The above-described embodiments are intended to facilitate understanding of the present disclosure and are not intended to limit the present disclosure. The present disclosure may be modified or improved without departing from the spirit thereof, and the present disclosure also includes equivalents thereof.

[0079] The present invention can also take the following forms: (1) A composition formula of Li ｘ MX ｙ M is one or more elements selected from the group 14 elements, X includes S, and Li, M, and X have a composition ratio of x:1:y; the crystal structure is a hexagonal crystal, and the volume of the unit cell of the hexagonal crystal is 0.0870 nm ３ Larger than 0.0880 nm ３ (2) The volume of the hexagonal unit cell is 0.08709 nm ３ 0.08795nm or more ３The solid electrolyte according to (1), which is the following: (3) The solid electrolyte according to (1) or (2), wherein M contains Sn. (4) The solid electrolyte according to (3), wherein M further contains Si. (5) The solid electrolyte according to any one of (1) to (4), wherein x satisfies 3.90<x<4.00 or 4.00<x≦4.10, and y satisfies 3.0≦y≦5.0. (6) A secondary battery comprising: a positive electrode; a negative electrode having a negative electrode active material layer containing a negative electrode active material; and an electrolyte layer containing the solid electrolyte according to any one of (1) to (5). (7) The secondary battery according to (6), wherein the negative electrode active material layer is a film made of a negative electrode active material. (8) The secondary battery according to (7), wherein a surface of the negative electrode active material layer facing the electrolyte layer is coated with the solid electrolyte. (9) A secondary battery comprising: a positive electrode, a negative electrode containing a particulate negative electrode active material and the solid electrolyte according to any one of (1) to (5), and an electrolyte layer. (10) The secondary battery according to (9), wherein at least a portion of the surface of the particles of the negative electrode active material is coated with the solid electrolyte.

[0080] REFERENCE SIGNS LIST 1, 1A to 1D Battery 10 Protective layer 20 Positive electrode 21 Positive electrode current collector layer 22 Positive electrode active material layer 30, 30A, 30B Negative electrode 31 Negative electrode current collector layer 32, 32A, 32B Negative electrode active material layer 32a Negative electrode active material 32b First solid electrolyte 32c Second solid electrolyte 40, 40A, 40B Electrolyte layer 41 First electrolyte layer 42 Second electrolyte layer 60 Side reinforcing portion

Claims

1. The formula is Li ｘ MX ｙ M is one or more elements selected from the group 14 elements, X includes S, and Li, M, and X have a composition ratio of x:1:y; the crystal structure is a hexagonal crystal, and the volume of the unit cell of the hexagonal crystal is 0.0870 nm ３ Larger than 0.0880 nm ３ Smaller solid electrolyte.

2. The volume of the hexagonal unit cell is 0.08709 nm ３ 0.08795nm or more ３ 2. The solid electrolyte of claim 1, wherein:

3. The solid electrolyte according to claim 1 or 2, wherein M contains Sn.

4. The solid electrolyte according to claim 3, wherein M further comprises Si.

5. A solid electrolyte according to any one of claims 1 to 4, wherein x satisfies 3.90<x<4.00 or 4.00<x≦4.10, and y satisfies 3.0≦y≦5.

0.

6. A secondary battery comprising: a positive electrode; a negative electrode having a negative electrode active material layer containing a negative electrode active material; and an electrolyte layer containing the solid electrolyte according to any one of claims 1 to 5.

7. The secondary battery according to claim 6, wherein the negative electrode active material layer is a film made of a negative electrode active material.

8. The secondary battery according to claim 7, wherein the surface of the negative electrode active material layer facing the electrolyte layer is coated with the solid electrolyte.

9. A secondary battery comprising: a positive electrode; a negative electrode containing a particulate negative electrode active material and the solid electrolyte according to any one of claims 1 to 5; and an electrolyte layer.

10. The secondary battery according to claim 9, wherein at least a portion of the surface of the particles of the negative electrode active material is coated with the solid electrolyte.

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