Solid electrolyte and secondary battery

Abstract

The present invention improves discharge characteristics 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 relationships 2.0<x<4.0 and 3.0≤y≤5.0 are satisfied. The crystal structure of the solid electrolyte is hexagonal.

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 discharge characteristics may be deteriorated.

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

[0006] The solid electrolyte according to one embodiment has a composition formula of Li ｘ MX ｙ wherein M is one or more elements selected from the Group 14 elements, X includes S, 2.0<x<4.0 and 3.0≦y≦5.0 are satisfied, and the crystal structure is a hexagonal system.

[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 the discharge characteristics while improving the water resistance.

[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 charge / discharge curves according to Examples 1 and 5 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 thereto, 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.

[0022] The average charging potential of the negative electrode active material is 1.0 V vs. Li ＋ It is preferable that the average charging potential of the negative electrode active material is 1.0 V vs. Li or less. This makes it possible to increase the output voltage of the battery. ＋ / Li or less, Li ４ SnS ４ On the other hand, in this embodiment, by using the solid electrolyte according to this embodiment as the first solid electrolyte in contact with the negative electrode active material described later, the average charging potential of the negative electrode active material becomes lower than the potential at which Li decomposes, and the solid electrolyte in contact with the negative electrode active material is reduced, resulting in a deterioration in discharge characteristics. ＋Even when the average charge potential of the negative electrode active material is less than 1 / Li, the discharge characteristics can be improved while suppressing reduction of the solid electrolyte in contact with the negative electrode active material. Here, the average charge potential of the negative electrode active material refers to the average negative electrode potential in a charge curve obtained in a charge test for a half cell using the negative electrode active material to be measured. Here, the average charge potential of the negative electrode active material can be measured by subjecting the half cell to charge measurement under the following conditions, for example, and measuring the average negative electrode potential from the start of charge to the end of charge in the obtained charge curve. Charge rate: 0.1 C Charging method: CC End-of-charge voltage: 0.03 V

[0023] The negative electrode active material layer 32 contains at least one of carbon (C), tin (Sn), and silicon (Si) as the negative electrode active material. When carbon is used as the negative electrode active material, the negative electrode active material is, for example, graphite or hard carbon, with graphite being preferred. This allows the average charging potential of the negative electrode active material to be 1.0 V vs. Li ＋ / Li or less, and as described above, the negative electrode active material comes into contact with the first solid electrolyte described below, which suppresses reduction of the solid electrolyte in contact with the negative electrode active material, thereby improving discharge characteristics.

[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 reduction resistance, thereby improving 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 Here, y satisfies 3.0≦y≦5.0. By setting y in this range, the crystal structure of the first solid electrolyte can be ４ SnS ４Since the structure can be the same as that of the negative electrode, water resistance can be improved. Here, 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. Here, when measuring y by ICP-OES, the ICP-OES measurement is performed using a sample obtained by extracting the solid electrolyte with water or methanol from the electrolyte layer or negative electrode removed from the battery.

[0028] The composition formula of the first solid electrolyte is Li ｘ MX ｙ In 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 improving reduction resistance, thereby improving discharge characteristics. M preferably contains Sn, and more preferably further contains Si. This further improves discharge characteristics. In addition, the composition formula of the first solid electrolyte, Li ｘ MX ｙ In the formula, X contains sulfur (S), which can improve discharge characteristics. Note that X is not limited to S alone, and may contain, for example, oxygen (O).

[0029] x satisfies 2.0<x<4.0, preferably 2.1≦x≦3.9, more preferably 2.5≦x≦3.9, and even more preferably 3.0≦x≦3.5. By setting x within this range, water resistance and reduction resistance can be improved, thereby improving discharge characteristics. x can be obtained by using the electrolyte layer or negative electrode removed from the battery as a sample and obtaining the molar ratio of Li to M from the spectral intensity measured by windowless energy dispersive X-ray spectroscopy (Window-Less Energy Dispersive X-ray Spectroscopy), inductively coupled plasma optical emission spectroscopy (ICP-OES), or glow discharge optical emission spectrometry (GD-OES). When measuring x by ICP-OES, the electrolyte layer removed from the battery or the solid electrolyte extracted from the negative electrode with water or methanol is used as a sample for ICP-OES measurement.

[0030] The crystal structure of the first solid electrolyte is hexagonal. This improves 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 after discharge 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.

[0031] 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, which has high reduction resistance, thereby improving discharge characteristics.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] As described above, the solid electrolyte (first solid electrolyte) according to this embodiment has the composition formula Li ｘ MX ｙwhere M is one or more elements selected from the Group 14 elements, X includes S, and the crystalline structure is hexagonal, satisfying 2.0<x<4.0 and 3.0≦y≦5.0. This improves water resistance and reduction resistance, thereby improving discharge characteristics.

[0038] In a preferred embodiment, 2.5≦x≦3.9 is further satisfied, which can improve water resistance and reduction resistance, thereby further improving discharge characteristics.

[0039] In a more desirable embodiment, 3.0≦x≦3.5 is further satisfied, which can improve water resistance and reduction resistance, thereby further improving discharge characteristics.

[0040] In a preferred embodiment, M contains Sn, which can improve the discharge characteristics.

[0041] In a preferred embodiment, M further contains Si, which can further improve the discharge characteristics.

[0042] 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 reduction resistance, thereby improving discharge characteristics.

[0043] 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 reduction resistance can be improved, and therefore the discharge characteristics can be improved.

[0044] 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, which has high reduction resistance, thereby improving discharge characteristics.

[0045] In a preferred embodiment, the average charging potential of the negative electrode active material is 1.0 V vs. Li ＋ / Li or less. This can improve the output characteristics.

[0046] In a preferred embodiment, the negative electrode active material contains at least one of C, Si, and Sn, which can improve the output characteristics.

[0047] 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.

[0048] 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.

[0049] 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).

[0050] 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 covered with the first solid electrolyte, which has high reduction resistance, thereby improving 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.

[0051] 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 discharge characteristics can be improved.

[0052] 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, the water resistance and reduction resistance can be improved, and therefore the discharge characteristics can be improved.

[0053] 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 reduction resistance, thereby improving discharge characteristics.

[0054] In a preferred embodiment, the average charging potential of the negative electrode active material is 1.0 V vs. Li ＋ / Li or less. This can improve the output characteristics.

[0055] In a preferred embodiment, the negative electrode active material contains at least one of C, Si, and Sn, which can improve the output characteristics.

[0056] 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.

[0057] In the third embodiment, the anode active material layer 32B includes a 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 covered with the first solid electrolyte 32b, thereby improving 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.

[0058] 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 reduction resistance, thereby further improving discharge characteristics.

[0059] 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.

[0060] Examples of the present embodiment will be described below, but the present embodiment is not limited to the following examples.

[0061] 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 1.95:1.0:2.0. 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 12 hours or more while heating 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 ３．９ SnS ３．９５ Furthermore, as a result of carrying out XRD measurement on the powder of the first solid electrolyte according to Example 1, the crystal structure of the first solid electrolyte according to Example 1 was found to be hexagonal.

[0062] <Charge / Discharge Measurement> A plurality of batteries (first to third batteries) having different negative electrode active materials were fabricated using the first solid electrolyte according to Example 1, and charge / discharge measurements were performed.

[0063] In the first battery, powdered graphite was used as the negative electrode active material. Here, the first battery is an example of the battery according to the second embodiment. The first battery was fabricated by the following method.

[0064] The electrolyte layer of the first battery was prepared by the following method. A zirconia cylinder having a diameter of 10 mm was filled with Li as the second solid electrolyte. ６ P.S. ５ 100 mg of Cl powder is 1 tf / (cm ２ The second electrolyte layer was then compressed at a pressure of 1 tf / (cm ２The mixture was pressed at a temperature of 100°C (min) to prepare a first electrolyte layer. In this way, an electrolyte layer for the first battery was prepared.

[0065] The negative electrode of the first battery was fabricated by the following method. First, the powder of the first solid electrolyte according to Example 1 and powdered graphite as the negative electrode active material were weighed out in a volume ratio of 1:1, and kneaded in a mortar to obtain a negative electrode mixture. Then, the fabricated negative electrode mixture was weighed out so that it contained an amount (16.1 g) of graphite as the negative electrode active material corresponding to a theoretical capacity of 6 mAh. This allowed the theoretical capacity of the first battery to be 6 mAh. The negative electrode mixture was then placed on the first electrolyte layer side of the electrolyte layer fabricated above, and a 3 tf / (cm ２ The negative electrode active material layer was prepared by compressing the negative electrode active material at a pressure of 1 / 2 min. This brought the surface of the negative electrode active material of the first battery into contact with the particles of the first solid electrolyte of Example 1. Thereafter, a stainless steel foil was attached to the negative electrode active material layer as a negative electrode current collector layer to prepare a negative electrode.

[0066] The first battery was assembled in the following manner. A counter electrode made of an In—Li alloy was attached to the second electrolyte layer side of the electrolyte layer. Then, a stainless steel foil was attached as a counter electrode current collector, and a current density of 1 tf / (cm ２ The laminate was pressed in the stacking direction at a pressure of 1 / 2 sq. min. Thus, a first battery according to Example 1 was fabricated.

[0067] The first battery was charged and discharged under the following conditions. More specifically, the fabricated battery was charged to an upper limit voltage using the following charge / discharge method and charge / discharge current, and then discharged to a lower limit voltage using the following charge / discharge method and charge / discharge current. Here, 1 C was defined as the theoretical capacity of the first battery, 6 mAh. The initial discharge capacity and charge / discharge efficiency were measured using this. Here, the initial discharge capacity was calculated per 1 g of negative electrode active material. The charge / discharge efficiency was calculated as the ratio of the discharge capacity at the initial discharge to the charge capacity at the initial charge. Charge / discharge method: CC Charge / discharge current: 0.05 C Lower limit voltage: -0.57 V (0.05 V vs. Li ＋ / Li) Upper limit voltage: 0.88V (1.5V vs. Li ＋ / Li)

[0068] The second battery was fabricated in the same manner as the first battery, except that powdered Si was used as the negative electrode active material instead of powdered graphite, and charge / discharge measurements were performed. In the process of fabricating the negative electrode for the second battery, the fabricated negative electrode mixture was weighed so that it contained an amount of Si (1.43 mg) as the negative electrode active material corresponding to a theoretical capacity of 6 mAh.

[0069] The third battery was fabricated in the same manner as the first battery, except that powdered Sn was used as the negative electrode active material instead of powdered graphite, and charge / discharge measurements were performed. In the negative electrode fabrication process for the third battery, the fabricated negative electrode mixture was weighed so that it contained an amount of Sn as the negative electrode active material (6.0 mg) corresponding to a theoretical capacity of 6 mAh.

[0070] 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 1.9:1.0:2.0. The composition of the first solid electrolyte according to Example 2 was Li ３．８ SnS ３．９０ Furthermore, as a result of carrying out XRD measurement on the powder of the first solid electrolyte according to Example 2, the crystal structure of the first solid electrolyte according to Example 2 was found to be hexagonal.

[0071] 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 1.75:1.0:2.0. The composition of the first solid electrolyte according to Example 3 was Li ３．５ SnS ３．７５ Furthermore, as a result of carrying out XRD measurement on the powder of the first solid electrolyte according to Example 3, the crystal structure of the first solid electrolyte according to Example 3 was found to be hexagonal.

[0072] 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 1.5:1.0:2.0. The composition of the first solid electrolyte according to Example 4 was Li ３．０ SnS ３．５０ Furthermore, as a result of carrying out XRD measurement on the powder of the first solid electrolyte according to Example 4, the crystal structure of the first solid electrolyte according to Example 4 was found to be hexagonal.

[0073] 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, and S powder in a molar ratio of 1.25:1.0:2.0. The composition of the first solid electrolyte according to Example 5 was Li ２．５ SnS ３．２５ Furthermore, as a result of carrying out XRD measurement on the powder of the first solid electrolyte according to Example 5, the crystal structure of the first solid electrolyte according to Example 5 was found to be hexagonal.

[0074] Comparative Example 1 In Comparative Example 1, 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 2.0:1.0:2.0. The composition of the first solid electrolyte according to Comparative Example 1 was Li ４．０ SnS ４．００ Furthermore, as a result of carrying out XRD measurement on the powder of the first solid electrolyte according to Comparative Example 1, the crystal structure of the first solid electrolyte according to Comparative Example 1 was found to be hexagonal.

[0075] 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 1.0:1.0:2.0. The composition of the first solid electrolyte according to Comparative Example 2 was Li ２．０ SnS ３．００Furthermore, as a result of carrying out XRD measurement on the powder of the first solid electrolyte according to Comparative Example 2, the crystal structure of the first solid electrolyte according to Comparative Example 2 was found to be hexagonal.

[0076] 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 1.0:1.0, 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 ２．０ SnS ３．００ Furthermore, as a result of carrying out XRD measurement on the powder of the first solid electrolyte according to Comparative Example 3, the crystal structure of the first solid electrolyte according to Comparative Example 3 was found to be a rock salt cubic crystal.

[0077] Table 1 shows the compositions of the first solid electrolytes according to Examples 1 to 5 and Comparative Examples 1 to 3, as well as the measurement results.

[0078]

[0079] As shown in Table 1, in Examples 1 to 5 and Comparative Examples 1 and 2 in which the first solid electrolyte was a hexagonal crystal, the discharge capacity was improved compared to Comparative Example 3 in which the first solid electrolyte was a cubic crystal. This shows that the discharge characteristics can be improved by using a hexagonal first solid electrolyte.

[0080] As shown in Table 1, Examples 1 to 5, where x is 2.0<x<4.0, had improved discharge capacities compared to Comparative Example 2, where x is 2.0, and Comparative Example 1, where x is 4.0. This shows that satisfying the condition 2.0<x<4.0 can improve discharge characteristics.

[0081] As shown in Table 1, Examples 1 to 5, where 2.5≦x≦3.9, had improved discharge capacities compared to Comparative Example 2, where x<2.5, and Comparative Example 1, where x>3.9. This shows that satisfying 2.5≦x≦3.9 can further improve discharge characteristics.

[0082] As shown in Table 1, Examples 3 and 4, where 3.0≦x≦3.5, had improved discharge capacities compared to Example 5 and Comparative Example 2, where x<3.0, and Example 1 and Comparative Example 1, where x>3.5. This shows that satisfying 3.0≦x≦3.5 can further improve discharge characteristics.

[0083] Fig. 6 shows charge / discharge curves for Examples 1 and 5 and Comparative Example 1. The charge / discharge curves in Fig. 6 are for a second battery in which the negative electrode active material is Si. As shown in Fig. 6, the discharge curves for the batteries of Examples 1 and 5 reach the discharge cutoff voltage of 1.5 V on the higher capacity side compared to Comparative Example 1. The improved discharge capacity in the batteries of Examples 1 and 5 is thought to be due to the better formation of a coating that suppresses decomposition of the solid electrolyte.

[0084] 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.

[0085] The present invention can also take the following forms: (1) A composition formula of Li ｘ MX ｙ(3) The solid electrolyte according to (2), wherein M is one or more elements selected from Group 14 elements, X includes S, and the crystalline structure is hexagonal, satisfying 2.0<x<4.0 and 3.0≦y≦5.0. (4) The solid electrolyte according to any one of (1) to (3), wherein M includes Sn. (5) The solid electrolyte according to (4), wherein M includes Si. (6) A secondary battery comprising: a positive electrode; a negative electrode having a negative electrode active material layer including a negative electrode active material; and an electrolyte layer including 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 the surface of the negative electrode active material layer facing the electrolyte layer is coated with the solid electrolyte. ＋ / Li or less. (10) The secondary battery according to any one of (6) to (9), wherein the negative electrode active material contains at least one of C, Si, and Sn. (11) A secondary battery comprising: a positive electrode; a negative electrode containing particulate negative electrode active material and the solid electrolyte according to any one of (1) to (5); and an electrolyte layer. (12) The secondary battery according to (11), wherein at least a portion of the surface of particles of the negative electrode active material is coated with the solid electrolyte. (13) The negative electrode active material has an average charging potential of 1.0 V vs Li ＋ (14) The secondary battery according to any one of (11) to (13), wherein the negative electrode active material contains at least one of C, Si, and Sn.

[0086] 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 empirical formula is Li ｘ MX ｙ A solid electrolyte represented as follows: M is one or more elements selected from the Group 14 elements, X contains S, satisfies 2.0 < x < 4.0 and 3.0 ≤ y ≤ 5.0, and has a hexagonal crystal structure.

2. The solid electrolyte according to claim 1, further satisfying 2.5 ≤ x ≤ 3.

9.

3. The solid electrolyte according to claim 2, further satisfying 3.0 ≤ x ≤ 3.

5.

4. The solid electrolyte according to any one of claims 1 to 3, wherein M contains Sn.

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

6. Positive electrode and, A negative electrode having a negative electrode active material layer containing a negative electrode active material, An electrolyte layer comprising a solid electrolyte according to any one of claims 1 to 3 A secondary battery equipped with these features.

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

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

9. The average charging potential of the negative electrode active material is 1.0V vs Li ＋ The secondary battery according to claim 6, wherein the value is less than or equal to / Li.

10. The secondary battery according to claim 6, wherein the negative electrode active material comprises at least one of C, Si, and Sn.

11. Positive electrode and, A negative electrode comprising particulate negative electrode active material and a solid electrolyte according to any one of claims 1 to 3, Electrolyte layer and A secondary battery equipped with these features.

12. The secondary battery according to claim 11, wherein at least a portion of the surface of the particles of the negative electrode active material is coated with the solid electrolyte.

13. The average charging potential of the negative electrode active material is 1.0V vs Li ＋ The secondary battery according to claim 11, wherein the value is less than or equal to / Li.

14. The secondary battery according to claim 11, wherein the negative electrode active material comprises at least one of C, Si, and Sn.