Solid electrolyte, positive electrode, and solid-state battery

Abstract

This solid electrolyte has: a first crystallite which is formed into a primary particle and has a first crystal structure; a second crystallite which is formed into the same primary particle as the first crystallite and has a second crystal structure that is different from the first crystal structure; and an amorphous phase. The first crystallite and the second crystallite each contain lithium, the first crystal structure is a hexagonal crystal structure, the second crystal structure is an orthorhombic crystal structure, and the crystallite size of the first crystallite and the crystallite size of the second crystallite are both 50 nm or less.

Description

Solid electrolyte, cathode and solid-state battery

[0001] The present invention relates to a solid electrolyte, a positive electrode, and a solid-state battery.

[0002] Patent Document 1, Patent Document 2 and Non-Patent Document 1 disclose solid-state batteries using a solid electrolyte instead of an organic solvent electrolyte.

[0003] JP 2022-139139 A JP 2015-069696 A

[0004] Misae Otoyama, Kentaro Kuratani, Hironori Kobayashi, "A systematic study on structure, ionic conductivity, and air-stability of xLi4SnS4・(1-x)Li3PS4 solid electrolytes" Ceramics International, 47 (2021) 28377-28383.

[0005] In such solid state batteries using a solid electrolyte, there is a demand for improving the ionic conductivity of the solid electrolyte.

[0006] An object of the present invention is to provide a solid electrolyte, a positive electrode, and a solid-state battery that can improve ionic conductivity.

[0007] A solid electrolyte according to one embodiment includes a first crystallite formed on a primary particle and having a first crystal structure, a second crystallite formed on the same primary particle as the first crystallite and having a second crystal structure that is a crystal structure different from the first crystal structure, and an amorphous phase, wherein the first crystallite and the second crystallite each contain lithium, the first crystal structure is a hexagonal crystal, and the second crystal structure is an orthorhombic crystal, and the crystallite size of the first crystallite and the crystallite size of the second crystallite are both 50 nm or less.

[0008] A positive electrode according to one embodiment includes a positive electrode active material layer provided in contact with the solid electrolyte.

[0009] A solid state battery according to one embodiment includes the above solid electrolyte, and a positive electrode and a negative electrode provided in contact with the solid electrolyte.

[0010] The solid electrolyte, positive electrode, and solid-state battery of the present invention can improve ionic conductivity.

[0011] Fig. 1 is a cross-sectional view schematically showing the configuration of a solid-state battery according to an embodiment. Fig. 2 is an explanatory diagram for explaining the configuration of a solid electrolyte. Fig. 3 is a table showing the configuration and ionic conductivity of solid electrolytes according to examples and comparative examples. Fig. 4 is a graph showing the measurement results of ionic conductivity according to examples and comparative examples.

[0012] Hereinafter, embodiments of the present disclosure will be described, but the present disclosure is not limited to these embodiments.

[0013] (Embodiment) Fig. 1 is a cross-sectional view schematically showing the configuration of a solid-state battery according to an embodiment. Fig. 2 is an explanatory diagram for explaining the configuration of a solid electrolyte. As shown in Fig. 1, a solid-state battery 10 according to an embodiment has a positive electrode 20, a negative electrode 30, and a solid electrolyte 40. The positive electrode 20 and the negative electrode 30 are stacked with the solid electrolyte 40 sandwiched between them. The solid-state battery 10 according to an embodiment is a secondary battery capable of repeated charge and discharge, and more specifically, a lithium-ion secondary battery.

[0014] The positive electrode 20 includes a positive electrode current collector 21 and a positive electrode active material layer 22 provided on one surface (upper surface) of the positive electrode current collector 21. The positive electrode current collector 21 of the positive electrode 20 is formed of a conductive material such as aluminum (Al). The positive electrode current collector 21 is not limited to aluminum and may be made of other conductive materials such as nickel or stainless steel.

[0015] The positive electrode active material layer 22 is a layer containing positive electrode active material particles. The positive electrode active material layer 22 is provided in contact with the solid electrolyte 40. The positive electrode active material is a positive electrode material capable of absorbing and releasing lithium ions, and for example, a lithium-containing composite oxide is used. Examples of the lithium-containing composite oxide include lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, and lithium iron phosphate. Alternatively, a part of the transition metal contained in these materials may be replaced with another metal.

[0016] The positive electrode active material layer 22 may contain a conductive agent and a binder as necessary. The conductive agent contained in the positive electrode active material layer 22 is, for example, a carbon material such as carbon black. The conductive agent is not limited to one type, and a mixture of multiple conductive materials may be used. Note that the conductive agent may be a metal material, a conductive polymer, or the like, as long as it is a material that is conductive.

[0017] The binder contained in the positive electrode active material layer 22 is, for example, polyvinylidene fluoride (PVDF). However, the binder is not limited to this, and may be a compound containing one or more of synthetic rubbers and polymeric compounds. Examples of synthetic rubbers include styrene-butadiene rubbers, fluorine-containing rubbers, and ethylene propylene dienes. Examples of polymeric compounds include polyvinylidene fluoride, polyimides, and carboxymethyl cellulose.

[0018] The positive electrode active material layer 22 may be configured to contain neither a binder nor a conductive additive, and to contain substantially no voids.

[0019] The negative electrode 30 has a negative electrode current collector 31 and a negative electrode active material layer 32 provided on one surface (lower surface) of the negative electrode current collector 31 .

[0020] The negative electrode current collector 31 of the negative electrode 30 is formed of a conductive material such as copper (Cu). The material of the negative electrode current collector 31 is not limited to copper, and may be other conductive materials such as nickel or stainless steel.

[0021] The negative electrode active material layer 32 of the negative electrode 30 contains a negative electrode active material and a binder. The negative electrode active material layer 32 is provided in contact with the solid electrolyte 40. The negative electrode active material may be, for example, a carbon material such as graphite, a silicon compound (SiO ｘ ) and the like. More specifically, the carbon material used for the negative electrode active material is, for example, at least one of graphitizable carbon, non-graphitizable carbon, and graphite (natural graphite and artificial graphite). The carbon material used for the negative electrode active material undergoes very little change in its crystal structure when absorbing and releasing lithium ions, and therefore can achieve high energy density and excellent cycle characteristics.

[0022] The binder contained in the negative electrode active material layer 32 is made of the same material as the binder contained in the above-mentioned positive electrode active material layer 22, for example, polyvinylidene fluoride (PVDF). However, the binder contained in the negative electrode active material layer 32 may be made of the same material as the binder contained in the positive electrode active material layer 22, or may be made of a different material.

[0023] The solid electrolyte 40 is provided between the positive electrode 20 and the negative electrode 30. One surface (upper surface) of the solid electrolyte 40 contacts the negative electrode 30 (negative electrode active material layer 32), and the other surface (lower surface) of the solid electrolyte 40 contacts the positive electrode 20 (positive electrode active material layer 22). The solid electrolyte 40 is a sintered body formed by sintering a solid electrolyte. The material of the solid electrolyte is formed from a material that allows ions to move between the positive electrode 20 and the negative electrode 30. The material of the solid electrolyte has a composition represented by the following formula (1):

[0024] Li ａ MX ｂ ...(1)

[0025] In formula (1), 3<a<5 and 3<b<5 are satisfied. M contains at least one of silicon (Si), antimony (Sb), aluminum (Al), tin (Sn), and lead (Pb). X contains at least one of sulfur (S) and selenium (Se).

[0026] 2, in the solid electrolyte 40, secondary particles are formed by aggregation of fine primary particles 45. In this embodiment, the average particle size of the primary particles 45 is, for example, 2 μm or less. The average particle size of the secondary particles is, for example, 0.5 μm or more and 30 μm or less.

[0027] 2, the solid electrolyte 40 includes a first crystallite 41, a second crystallite 42, and an amorphous phase 43 formed within the same primary particle 45. The first crystallite 41, the second crystallite 42, and the amorphous phase 43 each contain lithium. More specifically, the first crystallite 41, the second crystallite 42, and the amorphous phase 43 have a composition represented by the above formula (1).

[0028] The crystallite sizes (grain sizes) of the first crystallites 41 and the second crystallites 42 are both 50 nm or less. The first crystallites 41 have a first crystal structure that is different from the second crystallites 42. For example, the first crystallites 41 have a hexagonal crystal structure, and the second crystallites 42 have a rectangular crystal structure. The region between the first crystallites 41 and the second crystallites 42, i.e., the region of the primary particle 45 where no crystal structure is formed, is an amorphous phase 43.

[0029] As described above, in the solid electrolyte 40 of this embodiment, nanocrystals (first crystallites 41 and second crystallites 42) having at least two types of crystal structures are formed in the same primary particle, and therefore the interfaces between the first crystallites 41 and the second crystallites 42 are enlarged, thereby improving the ionic conductivity of the solid electrolyte 40.

[0030] Note that Figure 2 is merely a schematic illustration, and the number, shape, arrangement, etc. of the first crystallites 41 and the second crystallites 42 can be changed as appropriate. For example, in Figure 2, the first crystallites 41 and the second crystallites 42 are separated, but this is not limited to this, and the first crystallites 41 and the second crystallites 42 may be partially in contact. However, when the first crystallites 41 are in contact with each other, if a grain boundary exists between adjacent first crystallites 41 or if the adjacent first crystallites 41 have different orientations, the adjacent first crystallites 41 are defined as different crystallites. The same applies when the second crystallites 42 are in contact with each other. Furthermore, crystallites having three or more different crystal structures may be formed within the same primary particle 45.

[0031] (Examples) Fig. 3 is a table showing the configurations and ionic conductivities of solid electrolytes according to Examples and Comparative Examples. Fig. 4 is a graph showing the measurement results of ionic conductivities according to Examples and Comparative Examples. Note that the present invention is not limited by these Examples.

[0032] Example 1 The solid electrolyte according to Example 1 was prepared by the following method. First, raw material reagent Li ２ Weigh out the S:Sn:S so that the molar ratio is 2:1:2. ２ The total amount (g) of S, Sn and S was adjusted to 15 wt% in solution concentration by H２ After injecting O, the mixture is heated and stirred at 80°C for 24 hours to form a solution (Step 1).

[0033] Next, Li ２ After confirming that S, Sn, and S are dissolved, the resulting solution is vacuum dried at 120° C., thereby obtaining a powder (Step 2).

[0034] The obtained powder was heated in an inert gas at a temperature increase rate of 10° C. / min, and then heat-treated at 250° C. for 3 hours to obtain a solid electrolyte powder (Step 3).

[0035] (Calculation of ionic conductivity) 100 mg of the obtained solid electrolyte was pressed at room temperature at 294 MPa using a 10 mmφ ceramic insulating cylinder, and then subjected to a pressure of 98 MPa with an amplitude of 10 mV and a frequency of 10 ６ The electrochemical impedance spectrum was measured at -10 Hz, and the ionic conductivity was calculated from the resistance value obtained from the spectrum and the pellet thickness.

[0036] The graph shown in FIG. 4 shows the electrochemical impedance spectrum of the solid electrolyte, with the horizontal axis representing the real component Z′ of the impedance Z and the horizontal axis representing the imaginary component Z″ of the impedance Z. Note that FIG. 4 also shows the electrochemical impedance spectrum of the solid electrolyte of Example 1, as well as the electrochemical impedance spectra of Comparative Examples 1 and 3 for comparison.

[0037] As shown in Figure 4, the imaginary axis (capacitance) component of the electrochemical impedance spectrum is linearly extrapolated to calculate the resistivity (Z' (kΩcm)) of each solid electrolyte. The reciprocal of the calculated resistivity is the ionic conductivity (mScm －１ ) corresponds to

[0038] (Calculation of Crystallite Size) A diffraction pattern derived from the solid electrolyte was obtained by high-intensity X-ray diffraction measurement (XRD), and the crystallite size was calculated using the Scherrer method. Specifically, the crystallite size D was calculated based on the following formulas (2) and (3).

[0039] D=K×λ / (β×cosθ) ... (2) β=(B(sample) ２ -B(ref) ２) １／２ ... (3)

[0040] In the formulas (2) and (3), D represents the crystallite size, K represents the Scherrer constant, λ represents the wavelength of incident light, β represents the spread of the diffracted X-rays originating from the target crystallite, B(sample) represents the spread of the diffracted X-rays of the sample, B(ref) represents the spread of the diffracted X-rays of the reference material, and θ represents the Bragg angle.

[0041] In calculating the crystallite size of the examples and comparative examples, each solid electrolyte powder was filled into a quartz glass capillary with an inner diameter of 0.3 mm in an inert gas, sealed, and sampled, and XRD measurement was carried out at the Aichi Synchrotron Light Center BL5S2. Incident light of 15.5 keV was used.

[0042] The B(sample) of each of the first crystallite and the second crystallite was calculated using the following method. For the first crystallite, the 103 peak of the solid electrolyte (Hexagonal (hexagonal)) was connected at both ends to remove the background. Then, from the peak width at half the intensity of the peak bottom and top, the B(sample) of the first crystallite was calculated using the "Scherrer method". For the second crystallite, the 222 peak of the solid electrolyte (Orthorhombic (rectangular)) was connected at both ends to remove the background. Then, from the peak width at half the intensity of the peak bottom and top, the B(sample) of the second crystallite was calculated using the "Scherrer method".

[0043] B(ref) is lanthanum hexaboride LaB ６ The value of LaB was used for the analysis of the 103 peak of the solid electrolyte. ６ A straight line passing through the two points of the 210 peak at 0.0357 (2θ = 24.8697°) and the 211 peak at 0.0361 (2θ = 27.2881°) was calculated, and the value of B (ref) at the corresponding peak position was calculated. ６ A straight line passing through the two points of the 111 peak at 0.0367 (2θ=19.2013°) and the 200 peak at 0.0360 (2θ=22.2090°) was calculated, and the value of B(ref) at the corresponding peak position was calculated.

[0044] Example 2 Example 2 differs from Example 1 in the conditions of the heat treatment in step 3. Specifically, in Example 2, a solid electrolyte powder was obtained by performing a heat treatment at 275°C for 3 hours. The other production conditions and evaluation methods for the solid electrolyte were the same as those in Example 1.

[0045] Example 3 differs from Example 1 in the heat treatment conditions in step 3. Specifically, in Example 3, a solid electrolyte powder was obtained by heat treatment at 300°C for 3 hours. The other production conditions and evaluation methods for the solid electrolyte were the same as those in Example 1.

[0046] Example 4 Example 4 differs from Example 1 in the blending conditions of the raw material reagents in Step 1 and the heat treatment conditions in Step 3. Specifically, first, raw material reagent Li ２ The materials were weighed so that the molar ratio of S:Sn:Si:S was 2:0.85:0.15:2. In Example 4, a heat treatment was carried out at 275°C for 3 hours to obtain a solid electrolyte powder. The other production conditions and evaluation methods for the solid electrolyte were the same as those in Example 1.

[0047] Example 5 Example 5 differs from Example 4 in the blending conditions of the raw material reagents in step 1. Specifically, first, raw material reagent Li ２ The materials were weighed so that the molar ratio of S:Sn:Sb:S was 1.925:0.85:0.15:2.075. The other heat treatment conditions, solid electrolyte preparation conditions, and evaluation methods were the same as in Example 4.

[0048] Example 6 Example 6 differs from Example 4 in the blending conditions of the raw material reagents in step 1. Specifically, first, raw material reagent Li ２ The materials were weighed so that the molar ratio of S:Sn:Al:S was 1.775:0.85:0.15:2.225. The other heat treatment conditions, solid electrolyte preparation conditions, and evaluation methods were the same as in Example 4.

[0049] Example 7 Example 7 differs from Example 4 in the blending conditions of the raw material reagents in step 1. Specifically, first, raw material reagent Li ２The materials were weighed so that the molar ratio of S:Sn:S:Se was 2:1:1.9:0.1. The other heat treatment conditions, solid electrolyte preparation conditions, and evaluation methods were the same as in Example 4.

[0050] Comparative Examples Comparative Examples 1 to 4 differ from Example 1 described above in the conditions of the heat treatment in step 3. Specifically, in Comparative Example 1, a solid electrolyte powder was obtained by heat treatment at 350°C for 3 hours. In Comparative Example 2, a solid electrolyte powder was obtained by heat treatment at 400°C for 3 hours. In Comparative Example 3, a solid electrolyte powder was obtained by heat treatment at 200°C for 3 hours. In Comparative Example 4, a solid electrolyte powder was obtained by heat treatment at 500°C for 3 hours. The other production conditions and evaluation methods for the solid electrolytes were the same as those in Example 1.

[0051] Comparative Examples 5 to 8 differ from Examples 4 to 7 in the heat treatment conditions in Step 3. Specifically, in Comparative Examples 5 to 8, solid electrolyte powders were obtained by heat treatment at 400°C for 3 hours. Other production conditions and evaluation methods for the solid electrolytes were the same as those in Examples 4 to 7.

[0052] As shown in the table of FIG. 3, the composition of the solid electrolytes in Examples 1 to 3 was Li ４ SnS ４ That is, in the above formula (1), M is tin (Sn) and X is sulfur (S). The composition of the solid electrolyte of Example 4 is Li ４ Si ０．１５ Sn ０．８５ S ４ That is, in the above formula (1), M is silicon (Si) and tin (Sn), and X is sulfur (S).

[0053] The composition of the solid electrolyte of Example 5 was Li ３．８５ Sb ０．１５ Sn ０．８５ S ４ That is, in the above formula (1), M is antimony (Sb) and tin (Sn), and X is sulfur (S). The composition of the solid electrolyte of Example 6 is Li ３．５５ Al ０．１５ Sn ０．８５ S ４That is, in the above formula (1), M is aluminum (Al) and tin (Sn), and X is sulfur (S). The composition of the solid electrolyte of Example 7 is Li ４ SnS ３．９ Se ０．１ That is, in the above formula (1), M is tin (Sn), and X is sulfur (S) and selenium (Se).

[0054] All of Examples 1 to 7 have a first crystallite having a first crystal structure (Hexagonal) and a second crystallite having a second crystal structure (Orthorhombic). The crystallite size of the first crystallite (first crystallite size) and the crystallite size of the second crystallite (second crystallite size) are both 50 nm or less. More preferably, the crystallite size of the first crystallite and the crystallite size of the second crystallite are both 10 nm or more and 40 nm or less.

[0055] The solid electrolyte compositions of Comparative Examples 1 to 4 were: Li ４ SnS ４ The composition of the solid electrolyte of Comparative Example 5 is the same as that of Examples 1 to 3. ４ Si ０．５ Sn ０．５ S ４ The composition of the solid electrolyte of Comparative Example 6 is Li ３．８５ Sb ０．１５ Sn ０．８５ S ４ The composition of the solid electrolyte of Comparative Example 7 is Li ３．５５ Al ０．１５ Sn ０．８５ S ４ The composition of the solid electrolyte of Comparative Example 8 is Li ４ SnS ３．９ Se ０．１ and is the same as in Example 7.

[0056] However, in Comparative Examples 1, 2 and 5 to 8, the first crystallite having a first crystal structure (Hexagonal) and the second crystallite having a second crystal structure (Orthorhombic) are similar to the examples in configuration, but the crystallite size of at least one of the first crystallite and the second crystallite is larger than 50 nm. That is, in Comparative Example 1, the crystallite size of the second crystallite is 55 nm, and the crystallite size of at least the second crystallite is larger than 50 nm. In Comparative Example 2, the crystallite size of the first crystallite is 60 nm, the crystallite size of the second crystallite is 70 nm, and both the first crystallite size and the second crystallite size are larger than 50 nm. Also, in Comparative Examples 5 to 8, both the first crystallite size and the second crystallite size are larger than 50 nm.

[0057] In Comparative Example 3, first crystallites having the first crystal structure (hexagonal) are formed, and the second crystal structure is amorphous. In Comparative Example 4, only first crystallites having the first crystal structure (orthorhombic) are formed, and no second crystallites having the second crystal structure are formed.

[0058] The ionic conductivity of Examples 1 to 7 was 1.0 × 10 －１ 1.6 x 10 －１ In contrast, the ionic conductivities of Comparative Examples 1 to 8 were 9.0 × 10 －４ Above 5.0 x 10 －２ This shows that the solid electrolytes according to Examples 1 to 7 have better ionic conductivity than the comparative example.

[0059] As described above, the solid electrolytes according to the examples were shown to have better ionic conductivity than the solid electrolytes according to the comparative examples by controlling the crystallite size of both the first crystallite and the second crystallite to 50 nm or less.

[0060] Furthermore, the solid electrolytes according to the examples do not contain the LiPS-based solid electrolyte, which is a material with low water resistance as shown in Patent Documents 1 and 2 and Non-Patent Document 1, and therefore can improve ionic conductivity while ensuring water resistance.

[0061] Furthermore, since the solid electrolyte according to the embodiment can be synthesized by a liquid phase method, productivity can be improved compared to a process in which mechanical mixing and pulverization is performed by mechanical milling using a ball mill or the like.

[0062] In Examples 1 to 7, the solid electrolyte composition was Li ４ SnS ４ , Li ４ Si ０．１５ Sn ０．８５ S ４ , Li ３．８５ Sb ０．１５ Sn ０．８５ S ４ , Li ３．５５ Al ０．１５ Sn ０．８５ S ４ , Li ４ SnS ３．９ Se ０．１ Similarly, in other compositions represented by the above formula (1), the ionic conductivity can be improved by controlling the crystallite size of the first crystallite and the crystallite size of the second crystallite to 50 nm or less.

[0063] The above-described embodiment is intended to facilitate understanding of the present invention, and is not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit and scope of the present invention, and equivalents thereof are also included in the present invention.

[0064] The present disclosure may also have the following configurations.

[0065] (1) A solid electrolyte comprising: a first crystallite formed on a primary particle and having a first crystal structure; a second crystallite formed on the same primary particle as the first crystallite and having a second crystal structure different from the first crystal structure; and an amorphous phase, wherein the first crystallite and the second crystallite each contain lithium, the first crystal structure is a hexagonal crystal, and the second crystal structure is an orthorhombic crystal, and the crystallite size of the first crystallite and the crystallite size of the second crystallite are both 50 nm or less. (2) The solid electrolyte according to (1), having a composition represented by the following formula (1): Liａ MX ｂ ... (1) However, in formula (1), 3<a<5 and 3<b<5 are satisfied, M includes at least one of silicon (Si), antimony (Sb), aluminum (Al), tin (Sn), and lead (Pb), and X includes at least one of sulfur (S) and selenium (Se). (3) The solid electrolyte according to (1) or (2), wherein the average particle size of the primary particles is 2 μm or less. (4) A positive electrode having a positive electrode active material layer provided in contact with the solid electrolyte according to any one of (1) to (3). (5) A solid battery having the solid electrolyte according to any one of (1) to (3), and a positive electrode and a negative electrode provided in contact with the solid electrolyte.

[0066] REFERENCE SIGNS LIST 10 Solid-state battery 20 Positive electrode 21 Positive electrode current collector 22 Positive electrode active material layer 30 Negative electrode 31 Negative electrode current collector 32 Negative electrode active material layer 40 Solid electrolyte 41 First crystallite 42 Second crystallite 43 Amorphous phase 45 Primary particle

Claims

1. a first crystallite formed in the primary particle and having a first crystal structure; a second crystallite formed in the same primary particle as the first crystallite and having a second crystal structure that is a crystal structure different from the first crystal structure; an amorphous phase; the first crystallite and the second crystallite each contain lithium; the first crystal structure is hexagonal and the second crystal structure is orthorhombic; The crystallite size of the first crystallite and the crystallite size of the second crystallite are both 50 nm or less, It has a composition represented by the following formula (1): solid electrolyte. Li a MX b...(1) However, in formula (1), 3<a<5 and 3<b<5 are satisfied, M includes at least one of silicon (Si), antimony (Sb), aluminum (Al), tin (Sn), and lead (Pb); X includes at least one of sulfur (S) and selenium (Se).

2. The average particle size of the primary particles is 2 μm or less The solid electrolyte according to claim 1 .

3. A cathode active material layer provided in contact with the solid electrolyte according to claim 1 or 2. Positive electrode.

4. The solid electrolyte according to claim 1 or 2; A cathode and an anode are provided in contact with the solid electrolyte. solid state battery.