Sulfide-based solid electrolytes

Abstract

The present invention relates to a sulfide solid electrolyte which is used for lithium ion secondary batteries and contains an argyrodite crystal that contains Li, P, S and Ha, wherein: the argyrodite crystal satisfies the requirement (A) S, Cl and / or Br, and one or more elements other than S, Cl and Br are present in a free anion site, and / or the requirement (B) some P in a 4b site and some S in a 16e site that is adjacent to the 4b site are respectively substituted by other elements; and ((1 / χ(S)) × [S2-] + (1 / χ(O)) × [O2-] + (1 / χ(Br)) × [Br-] + (1 / χ(Cr)) × [Cl-] + (1 / χ(F)) × [F-]) ≤ 0.36 and [S2-] + [O2-] + [Br-] + [Cl-] + [F-] = 1 are satisfied.

Description

Technical Field

[0001] The present invention relates to a sulfide-based solid electrolyte used in a lithium-ion secondary battery.

Background Art

[0002] Lithium-ion secondary batteries are widely used in portable electronic devices such as mobile phones and notebook computers. Conventionally, liquid electrolytes have been used in lithium-ion secondary batteries, but there have been concerns about liquid leakage and ignition, and it has been necessary to increase the size of the case for safety design. In addition, improvements have been desired in terms of the short battery life and narrow operating temperature range of lithium-ion secondary batteries.

[0003] On the other hand, all-solid-state lithium-ion secondary batteries using a solid electrolyte as the electrolyte of the lithium-ion secondary battery have attracted attention because they can be expected to improve safety, high-speed charging and discharging, and miniaturization of the case.

[0004] Solid electrolytes are roughly classified into sulfide-based solid electrolytes and oxide-based solid electrolytes. Sulfide ions constituting the sulfide-based solid electrolyte have a larger polarization rate than oxide ions constituting the oxide-based solid electrolyte and exhibit high lithium-ion conductivity. As the sulfide-based solid electrolyte, LGPS-type crystals such as Li 10 GeP2S 12 and the like, argyrodite-type crystals such as Li6PS5Cl, and LPS crystallized glass such as Li7P3S 11 are known.

[0005] As an example in which a sulfide-based solid electrolyte containing an argyrodite-type crystal is disclosed, Patent Document 1 can be cited. The sulfide-based solid electrolyte disclosed in Patent Document 1 has a crystal structure belonging to the cubic space group F-43m and has a composition formula: Li 7-x PS[[ID=3S]] 6-x Ha x (Ha is Cl or Br) (x = 0.2 to 1.8) and contains a compound represented by L * a * b *The color system's lightness L value is 60.0 or higher. This is intended to improve charge / discharge efficiency and cycle characteristics by increasing lithium-ion conductivity and decreasing electronic conductivity.

[0006] When using such sulfide-based solid electrolytes in lithium-ion secondary batteries, if a positive electrode such as LiCoO2 or a ternary system of nickel, manganese, and cobalt called NMC is used, the output characteristics and discharge capacity decrease with repeated charging and discharging. This is due to an increase in the interfacial resistance between the positive electrode active material and the sulfide-based solid electrolyte. In contrast, Non-Patent Documents 1 and 2 disclose that the above-mentioned interfacial resistance can be reduced by coating the surface of the positive electrode active material, LiCoO2, with lithium niobate (LiNbO3), thereby improving the performance of all-solid-state lithium-ion secondary batteries. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2015 / 012042 [Non-patent literature]

[0008] [Non-Patent Document 1] Solid State Ionics,Volume 225(2012) Page 594-597 [Non-Patent Document 2] Chemistry of Materials,22(3)(2010) Page 949-956 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The ideal thickness of the surface coating on the positive electrode active material using coating agents such as LiNbO3 is considered to be 7-10 nm, but it is difficult to apply such an extremely thin coating uniformly and reproducibly. Furthermore, spinel-type crystalline LiNi, known as a high-potential positive electrode active material, is also a problem.x Mn 2-x When using O4 or similar materials, coating agents such as LiNbO3 may not be suitable for surface coating, as they may decompose under high electromotive force.

[0010] Therefore, the present invention aims to provide a sulfide-based solid electrolyte that suppresses the deterioration of battery characteristics when repeatedly charged and discharged, without coating the surface of positive electrode active materials such as LiCoO2 or NMC. [Means for solving the problem]

[0011] As a result of diligent research, the inventors of this invention have found that by adopting an argyrodite-type crystal, which has the characteristic of being able to be substituted with various elements as a sulfide-based solid electrolyte, and by solid-solving components that could not be conventionally dissolved, the above problems can be solved, and thus the present invention has been completed.

[0012] In other words, the present invention relates to the following [1] to [8]. [1] A sulfide-based solid electrolyte used in lithium-ion secondary batteries, comprising an argyrodite-type crystal containing Li, P, S, and Ha, wherein Ha is at least one element selected from the group consisting of F, Cl, Br, and I, and includes at least one of Cl and Br, and the argyrodite-type crystal satisfies at least one of the following: (A) the free anion sites contain S, at least one of Cl and Br, and one or more elements different therefrom, and (B) a portion of the P at the 4b site and a portion of the S at the 16e site adjacent to the P at the 4b site are each substituted with other elements, and the argyrodite-type crystal has a free anion content [S 2- ],O 2- ], [Br - ] [Cl - ] and [F - ], and their electronegativity χ (S) , χ (O) , χ (Br) , χ (Cl) and χ (F) but, {(1 / χ(S) )×[S 2- ]+(1 / χ (O) )×[O 2- ]+(1 / χ (Br) )×[Br - ]+(1 / χ (Cl) )×[Cl - ]+(1 / χ (F) )×[F - ]}≦0.36, and [S 2- ]+[O 2- ]+[Br - ]+[Cl - ]+[F - A sulfide-based solid electrolyte that satisfies the relationship ]=1. [2] The sulfide-based solid electrolyte according to [1], wherein the argyrodite-type crystal further comprises at least one of O and F. [3] Content of the free anions [S 2- ],O 2- ], [Br - ] [Cl - ] and [F - ], and their electronegativity χ (S) , χ (O) , χ (Br) , χ (Cl) and χ (F) but, {(1 / χ (S) )×[S 2- ]+(1 / χ (O) )×[O 2- ]+(1 / χ (Br) )×[Br - ]+(1 / χ (Cl) )×[Cl - ]+(1 / χ (F) )×[F - ]}≦0.34 A sulfide-based solid electrolyte according to [1] or [2] above, satisfying the relationship. [4] The argyrodite-type crystal satisfies (B) above, and the average bond distance between the 4b site and the adjacent 16e site is 2.07 Å or more, the sulfide-based solid electrolyte according to any one of [1] to [3] above. [5] The argyrodite-type crystal satisfies (B) above, and the element M that is substituted for part of the P at the 4b site is PS4 3- A sulfide-based solid electrolyte according to any one of [1] to [4] above, which forms an MS4 tetrahedral structure with a larger ionic radius than [1]. [6] The sulfide-based solid electrolyte according to [5], wherein the element M in the MS4 tetrahedron structure is at least one of Si and Sn. [7] The argyrodite-type crystal satisfies (A) above, and the content of the free anions is 0 < {[Br - ] / ([F - ]+[Cl - ]+[Br - A sulfide-based solid electrolyte according to any one of [1] to [6] above, satisfying the relationship ])}<2. [8] The sulfide-based solid electrolyte according to any one of [1] to [7], wherein the electromotive force of the lithium-ion secondary battery is 4.3V or more. [Effects of the Invention]

[0013] The sulfide-based solid electrolyte according to the present invention suppresses the deterioration of battery characteristics during repeated charging and discharging without coating the surface of the positive electrode active material with LiNbO3 or the like. Therefore, when using the above sulfide-based solid electrolyte in a lithium-ion secondary battery, the process of uniformly and reproducibly applying a very thin coating to the surface of the positive electrode active material becomes unnecessary. Furthermore, spinel-type crystal LiNi, known as a high-potential positive electrode active material, can also be used. x Mn 2-x Even when using O4 or similar materials, it can be applied without concern about the decomposition of the coating agent. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 shows the XRD patterns of sulfide-based solid electrolytes for Example 1 and Example 2, where the peak at 2θ = 28.5° is a peak originating from the Si internal standard. [Figure 2] Figure 2 shows the XRD pattern of the sulfide-based solid electrolyte in Example 8. [Figure 3]Figure 3 shows the charge-discharge curve obtained from the battery evaluation in Example 1. [Modes for carrying out the invention]

[0015] The present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be modified and implemented as appropriate without departing from the spirit of the invention. Furthermore, the "~" indicating a numerical range is used to mean that the numbers written before and after it are included as the lower limit and upper limit.

[0016] <Sulfide solid electrolyte> The sulfide-based solid electrolyte (hereinafter sometimes simply referred to as "solid electrolyte") according to this embodiment is used in lithium-ion secondary batteries and contains argyrodite-type crystals containing Li, P, S, and Ha. Ha is at least one element selected from the group consisting of F, Cl, Br, and I, and includes at least one of Cl and Br.

[0017] The argyrodite-type crystal in this embodiment satisfies at least one of the following conditions (A) and (B). (A) The free anion site contains S, at least one of Cl and Br, and one or more elements other than those. (B) A portion of the P at the 4b site and a portion of the S at the 16e site adjacent to the P at the 4b site are substituted with other elements.

[0018] Furthermore, the argyrodite-type crystal in this embodiment has a free anion content of [S 2- ],O 2- ], [Br - ] [Cl - ] and [F - ] and the electronegativity χ of those anions (S) , χ (O) , χ (Br) , χ (Cl) and χ (F) When expressed in this way, the following relationship is satisfied. {(1 / χ (S) )×[S2- +(1 / χ (O) )×[O 2- +(1 / χ (Br) )×[Br - +(1 / χ (Cl) )×[Cl - +(1 / χ (F) )×[F -} ≤ 0.36 and [S 2- +[O 2- +[Br - +[Cl - +[F - = 1 After that, {(1 / χ (S) )×[S 2- +(1 / χ (O) )×[O 2- +(1 / χ (Br) )×[Br - +(1 / χ (Cl) )×[Cl - +(1 / χ (F) )×[F -} may be referred to as the "anion parameter."

[0019] When the argyrodite-type crystal satisfies at least one of the above (A) and (B) and the anion parameter is 0.36 or less, it is possible to suppress a decrease in battery characteristics when charging and discharging are repeated without coating the surface of the positive electrode active material with LiNbO3 or the like.

[0020] First, the case where the above (A) is satisfied and the anion parameter is small will be described. The free anion site in the above (A) refers to an anion that is not covalently bonded to a cation. When the argyrodite-type crystal is cubic, the 4a site and the 4d site correspond. Also, although details will be described later, when the argyrodite-type crystal is rhombohedral, the 1a site and the 3b site correspond to the free anion site.

[0021] The proportion of free anions present at free anion sites can be determined by synchrotron X-ray diffraction (XRD) measurements. Specifically, the elements and occupancy rates of each site are determined by performing structural refinement analysis using the Rietveld method on the pattern obtained from synchrotron XRD measurements. Furthermore, by determining the content of each element and their totals through compositional analysis using methods such as ICP emission spectrometry, atomic absorption spectrometry, and ion chromatography, and then refining the crystal structure using the Rietveld method based on these values, a more accurate analysis can be achieved.

[0022] The one or more elements present in this free anion site, different from S, Cl, and Br, include, for example, O, F, Se, and I. However, from the viewpoint of having high electronegativity and reducing the anion parameter, at least one of O and F is preferred, and F is more preferred.

[0023] For example, if oxygen is present at a free anion site, it will be substituted with sulfur. Also, if fluorine is present at a free anion site, it will be substituted with chlorine or brinol. When such substitution is performed, the anion parameter becomes smaller, and the deterioration of battery characteristics can be suppressed without coating the surface of the positive electrode active material with LiNbO3 or the like.

[0024] The reason is unclear, but I think it's as follows: When a lithium-ion secondary battery using an uncoated positive electrode active material is charged and discharged, components diffuse to each other at the interface between the positive electrode active material and the sulfide-based solid electrolyte. As a result, oxidation reactions occur on the sulfide-based solid electrolyte side, more precisely, reactions between oxygen in the positive electrode active material and sulfides in the solid electrolyte. One of the causes of oxidation reactions is particularly sulfur. 2-It is possible that these are free anions, that is, unstable ions that are not covalently bonded to other elements. By making these free anions from elements with high electronegativity as much as possible, the gap between the upper end of the valence band and the lower end of the conduction band will increase, making oxidation reactions less likely to occur at high potentials. Consequently, a good SEI (Solid Electrolyte Interface) will be formed at the interface between the positive electrode active material and the sulfide-based solid electrolyte, thereby suppressing the deterioration of battery performance.

[0025] From the viewpoint of obtaining a suitable lithium-ion conductivity while achieving the above effects, the content of free anions should be 0 < {[Br - ] / ([F - ]+[Cl - ]+[Br - It is preferable that the relationship {[Br - ] / ([F - ]+[Cl - ]+[Br - The value represented by ])} is more preferably greater than 0.3, even more preferably greater than 0.5, more preferably less than 1.5, and even more preferably less than 1.2.

[0026] Next, we will explain the case where condition (B) above is satisfied and the anion parameter is small. The other element M that replaces part of P at site 4b of (B) above is preferably at least one element selected from the group consisting of metallic and metalloid elements of groups 2 to 14 of the periodic table, PS4 3- Elements that form an MS4 tetrahedron structure with a larger ionic radius are more preferable. Examples of elements M that form an MS4 tetrahedral structure with a large ionic radius include Si, Sn, Al, V, Ti, Zr, Sb, and Ge. Among these, Si, Sn, V, Ge, and Zr are even more preferred due to their high valency and ease of substitution with P.

[0027] The other elements that substitute for part of the S at the 16e site in (B) above are preferably O or Se, with O being even more preferred from the viewpoint of having high electronegativity and reducing the anionic parameter.

[0028] The other elements M that replace part of the P at the 4b site and the other elements that replace part of the S at the 16e site, as well as the proportion of free anions mentioned above, can be determined by synchrotron XRD measurements and Rietveld analysis.

[0029] Thus, when condition (B) above is met, the deterioration of battery characteristics can be suppressed even without a coating agent. Furthermore, when condition (B) above is met, the effects of the present invention are achieved even if the anion parameter value is slightly higher compared to the case where condition (A) above is met but condition (B) above is not met.

[0030] The reason is unclear, but I think it's as follows: PS4 3- The inclusion of MS4 tetrahedral structures with different structures and ionic radii within the crystal structure results in a more homogeneous charge distribution in argyrodite-type crystals. As a result, particularly S 2- We believe that unstable anions that are not covalently bonded to other elements, such as those mentioned above, can be stabilized, leading to the formation of a good SEI at the interface between the positive electrode active material and the sulfide-based solid electrolyte, thereby suppressing the deterioration of battery performance.

[0031] The above effect is due to the ionic radius of the MS4 tetrahedron structure being PS4 3- This is more pronounced when the size is larger than the structure. Therefore, the element M that replaces part of the P at the 4b site is, as mentioned earlier, PS4 3- Elements that form an MS4 tetrahedral structure with a larger ionic radius than SiS4 are preferred, and element M is more preferably at least one of Si and Sn. That is, the MS4 tetrahedral structure is SiS4 4- and SnS4 4- At least one of these is more preferable, and in this case, it is preferable that a portion of S is further replaced with O, Se, etc., to satisfy (B) above.

[0032] The ionic radius of the MS4 tetrahedron structure is PS4 3- In cases where the bond distance is greater than that of the structure, the average bond distance between the 4b site and the adjacent 16e site is preferably 2.07 Å or greater, more preferably 2.08 Å or greater, and even more preferably 2.10 Å or greater. Furthermore, from the viewpoint of maintaining the crystal structure, the average bond distance is preferably 2.20 Å or less, more preferably 2.17 Å or less, and even more preferably 2.15 Å or less. Note: PS4 3- The average bond distance between the 4b site and the adjacent 16e site in the structure is 2.04 Å. This average bond distance was determined by synchrotron XRD measurements and Rietveld analysis.

[0033] In this embodiment, the argyrodite-type crystal preferably further contains at least one of O and F, in addition to Li, P, S, and at least one of Cl and Br. From the viewpoint of (A) above, both O and F are preferred as elements contained in the free anion site, with F being more preferred. Furthermore, from the viewpoint of (B) above, it is preferable to include O as the element in which a portion of the S at the 16e site adjacent to the P at the 4b site is substituted. Furthermore, from the perspective of reducing the anion parameter, the electronegativity χ of electronegativity S (S) The electronegativity of O, Br, and Cl is χ, which has a lower value than this. (Br) , χ (Cl) It is preferable to include F, which is a lower value than [a certain value].

[0034] {(1 / χ (S) )×[S 2- ]+(1 / χ (O) )×[O 2- ]+(1 / χ (Br) )×[Br - ]+(1 / χ (Cl) )×[Cl - ]+(1 / χ (F) )×[F - The anion parameter, represented by ]}, is the sum of the products of the ratio of free anion content and the reciprocal of the electronegativity of each element. The lower the value of this anion parameter, the more the argyrodite crystal is composed of elements with high electronegativity. This increases the gap between the upper end of the valence band and the lower end of the conductor, making oxidation reactions less likely to occur at high potentials.

[0035] In this embodiment, the anion parameter value should be 0.36 or less, preferably 0.35 or less, and more preferably 0.34 or less. Furthermore, from the viewpoint of forming argyrodite-type crystals, the anion parameter value is preferably 0.30 or higher, and more preferably 0.32 or higher. The electronegativity of each element in the anion parameter is χ (S) =2.5, χ (O) =3.5, χ (Br) =2.8, χ (Cl) =3.0, χ (F) = 4.0.

[0036] The argyrodite crystal structure can be analyzed from the XRD pattern of a general-purpose instrument, but from the viewpoint of analytical precision, it is preferable to analyze it from the synchrotron radiation XRD pattern. The arrangement of each element in the crystal structure can be determined by refining the crystal structure using the Rietveld method on the XRD pattern measured with synchrotron X-rays. Furthermore, the content of each element and their total can be determined by compositional analysis using methods such as ICP emission spectrometry, atomic absorption spectrometry, and ion chromatography. By refining the crystal structure using the Rietveld method based on these values, the crystal composition can be determined with greater accuracy.

[0037] In the XRD pattern when the source is Cu-Kα radiation, the presence of peaks at 2θ = 15.7 ± 0.8° and 30.2 ± 0.8° indicates that the crystal is of the cubic argyrodite type. In addition to the above, it is preferable that the XRD pattern also has a peak at 2θ = 18.0 ± 0.8°, and even more preferable that it has a peak at 2θ = 25.7 ± 0.8°.

[0038] Furthermore, in the XRD pattern when the source is Cu-Kα radiation, peaks are present at 2θ = 15.8 ± 0.8°, 25.8 ± 0.8°, and 30.3 ± 0.8°, and at least two of these three peaks are split into two or more, indicating that the crystal is of the rhombohedral argyrodite type. In addition to the above, it is preferable that the XRD pattern has an unsplit peak at 2θ = 18.1 ± 0.8°, and even more preferable that it has a split peak at 2θ = 31.8 ± 0.8°.

[0039] The ratio of elemental content (at%) that constitutes an argyrodite crystal, that is, the ratio of elemental content contained in an argyrodite crystal, is given by Li α PS β Ha γ When expressed as such, it is preferable that the relationship 5 < α < 7, 3 < β < 6, and 0 < γ < 2.5 is satisfied, as this makes it easier for the crystal to be of the argyrodite type. It is more preferable that such elemental ratios satisfy the relationship 5.1 < α < 6.3, 3.5 < β < 5.3, and 0.7 < γ < 2.0, and even more preferable that they satisfy the relationship 5.2 < α < 6.2, 3.7 < β < 5.2, and 0.8 < γ < 1.9. In other words, for α, a value greater than 5 is preferable, a value greater than 5.1 is more preferable, a value greater than 5.2 is even preferable, a value less than 7 is preferable, a value less than 6.3 is more preferable, and a value less than 6.2 is even preferable. Regarding β, a value greater than 3 is preferred, a value greater than 3.5 is more preferred, a value greater than 3.7 is even more preferred, a value less than 6 is preferred, a value less than 5.3 is more preferred, and a value less than 5.2 is even more preferred. For γ, a value greater than 0 is preferred, a value greater than 0.7 is more preferred, a value greater than 0.8 is even more preferred, a value less than 2.5 is preferred, a value less than 2.0 is more preferred, and a value less than 1.9 is even more preferred.

[0040] The preferred crystal structure of argyrodite-type crystals is cubic, such as F-43m, but the rhombohedral crystal mentioned above is also acceptable. Furthermore, crystals with reduced symmetry, such as hexagonal, tetragonal, orthorhombic, monoclinic, and even triclinic crystals with further reduced symmetry, may also exist.

[0041] The halogen element represented by Ha is at least one selected from the group consisting of F, Cl, Br, and I, but since the crystal tends to be of the argyrodite type, it contains at least one of Cl and Br, preferably Cl, and more preferably elemental Cl or a mixture of Cl and Br.

[0042] When Ha contains Cl and Br, and the Cl content in the argyrodite-type crystal is c1 (at%) and the Br content is c2 (at%), the ratio of the content expressed as (c1 / c2) is preferably 0.1 or higher, more preferably 0.3 or higher, and even more preferably 0.5 or higher. Furthermore, (c1 / c2) is preferably 10 or less, more preferably 3 or less, and even more preferably 1.6 or less. When (c1 / c2) satisfies the above range, the interaction between lithium ions and halide ions is weakened, and the lithium ion conductivity of the sulfide-based solid electrolyte tends to be good. This is thought to be due to the mixed anion effect, which weakens the interaction between cations and anions due to the mixing of bromide ions, which have a larger ionic radius than chloride ions. In addition, when (c1 / c2) satisfies the above range, the cycle characteristics of lithium-ion secondary batteries tend to be improved.

[0043] Furthermore, if Ha contains Cl and Br, the ratio of the element content (at%) that constitutes the argyrodite type crystal is Li α PS β Cl γ1 Br γ2When expressed as such, γ1 is preferably 0.1 or higher, more preferably 0.3 or higher, even more preferably 0.5 or higher, and also preferably 1.5 or lower, more preferably 1.4 or lower, and even more preferably 1.3 or lower. γ2 is preferably 0.1 or higher, more preferably 0.3 or higher, even more preferably 0.5 or higher, and also preferably 1.9 or lower, more preferably 1.6 or lower, and even more preferably 1.4 or lower. By satisfying the above ranges for γ1 and γ2, the proportion of halide ions in the crystal is optimized, and a stable argyrodite-type crystal can be obtained while reducing the interaction between anions and lithium ions in the crystal. This tends to result in good lithium ion conductivity of the sulfide-based solid electrolyte. Furthermore, when γ1 and γ2 satisfy the above ranges, the cycle characteristics of the lithium-ion secondary battery tend to improve. Here, it is preferable that α, β, and (γ1+γ2) satisfy the same relationships as α, β, and γ described above.

[0044] From the viewpoint of obtaining good lithium-ion conductivity when a sulfide-based solid electrolyte layer is formed into a battery using a sulfide-based solid electrolyte, a smaller crystallite size is preferable for the crystals constituting the crystalline phase. Specifically, a crystallite size of 1000 nm or less is preferable, 500 nm or less is more preferable, and 250 nm or less is even preferable. There is no particular lower limit to the crystallite size, but it is usually 5 nm or more. Crystallite size can be calculated using the full width at half maximum (FMAX) of the peaks in the XRD pattern and Scherrer's equation. Furthermore, the crystallite size can be more precisely determined by refining the crystal structure using the Rietveld method.

[0045] From the viewpoint of achieving good battery characteristics, the content of argyrodite-type crystals relative to the total components constituting the sulfide-based solid electrolyte is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more. Furthermore, there is no particular upper limit to the above content, and it may be 100% by mass, but generally it is 99% by mass or less. The proportion of argyrodite-type crystals can be calculated by adding an internal standard material, measuring the peak intensity using XRD or neutron scattering, and then comparing it with the peak intensity of the internal standard material. Argyrodite-type crystals may contain two or more different crystal structures. The content of argyrodite-type crystals includes elements other than S, Cl, and Br present at the free anion sites, as well as other elements substituted for P at the 4b site and S at the 16e site. Furthermore, the content of each element and their total amounts can be determined by compositional analysis using methods such as ICP emission spectrometry, atomic absorption spectrometry, and ion chromatography.

[0046] In addition to argyrodite-type crystals, other materials that may be included in the solid electrolyte include amorphous components that can become argyrodite-type, oxide anions, and impurity crystalline phases such as Li3PS4, Li4P2S6, Li2S, and LiHa (where Ha is at least one halogen element selected from F, Cl, Br, and I). Impurity crystalline phases such as Li3PS4, Li4P2S6, Li2S, and LiHa may be present as long as they do not affect lithium-ion conductivity or battery characteristics. For example, the amount should be 15% by mass or less relative to the sulfide-based solid electrolyte, preferably 10% by mass or less, and more preferably 5% by mass or less. If impurity crystalline phases are present, it is better to perform crystal structure analysis after subtracting such impurity crystalline phases. However, if the amount of impurity crystalline phases is small, it may not significantly affect the analysis results.

[0047] The sulfide-based solid electrolyte preferably has a lithium ion conductivity of 1 mS / cm or more, more preferably 2 mS / cm or more, and even more preferably 4 mS / cm or more at 25°C when molded under pressure at 380 MPa; the higher the conductivity, the better. The lithium ion conductivity is determined from the Nyquist plot obtained by AC impedance measurement.

[0048] (Lithium-ion rechargeable battery) In this embodiment, the sulfide-based solid electrolyte, when used in lithium-ion secondary batteries, forms a good SEI even without surface coating of the positive electrode active material, thereby suppressing the deterioration of battery characteristics when repeated charging and discharging occurs. In charge-discharge tests, the initial characteristic expressed as (discharge capacity in the first cycle / charge capacity in the first cycle) is preferably greater than 0.60, more preferably 0.65 or higher, even more preferably 0.70 or higher, and the higher the value, the better. Furthermore, in charge-discharge tests, the capacity retention rate, expressed as (discharge capacity after 5 cycles / discharge capacity after 1 cycle) × 100 (%), is preferably over 86%, more preferably 88% or higher, even more preferably 90% or higher, and the higher the value, the better.

[0049] When sulfide-based solid electrolytes are used in lithium-ion secondary batteries, they form a solid electrolyte layer together with other components such as binders as needed. Conventionally known binders and other components are used. The content of sulfide-based solid electrolytes relative to the entire solid electrolyte layer is preferably 80% by mass or more, and more preferably 90% by mass or more.

[0050] Conventional methods are also used for forming the solid electrolyte layer. As an example of wet molding, the components constituting the solid electrolyte layer can be dispersed or dissolved in a solvent to form a slurry, which can then be coated in layers, i.e., in a sheet form, dried, and optionally pressed to form the solid electrolyte layer. If necessary, heat may be applied to remove the binder. The thickness of the solid electrolyte layer can be easily adjusted by adjusting the amount of slurry applied.

[0051] Alternatively, instead of wet molding, a solid electrolyte layer may be formed by dry press molding of a sulfide-based solid electrolyte powder on the surface of the positive or negative electrode. Alternatively, a solid electrolyte layer may be formed on another substrate and then transferred to the surface of the positive or negative electrode.

[0052] Sulfide-based solid electrolytes may be mixed with positive electrode active material or negative electrode active material and used as a positive electrode layer or negative electrode layer. Conventionally known materials are used for the positive electrode active material or negative electrode active material, current collector, binder, conductive additive, etc., used in the positive electrode layer or negative electrode layer.

[0053] In particular, for the positive electrode active material, it is preferable to use LiCoO2 or NMC, which have conventionally been surface-coated with LiNbO3 or the like, as this allows for greater enjoyment of the effects achieved by the present invention. Furthermore, positive electrode active materials that operate at higher potentials than conventional materials and are difficult to apply surface coatings such as LiNbO3 to are also preferable because they can better benefit from the effects of the present invention. Specifically, spinel-type crystalline LiNi, known as a high-potential positive electrode active material, is preferable. x Mn 2-x A positive electrode active material generally referred to as 5V class, such as O4, is preferred. As an indicator of a positive electrode active material that operates at a higher potential than conventional materials, sulfide-based solid electrolytes are preferably used in lithium-ion secondary batteries with an electromotive force of 4.3V or higher.

[0054] A lithium-ion secondary battery using a sulfide-based solid electrolyte includes the above-mentioned solid electrolyte layer, a positive electrode layer, and a negative electrode layer. The casing material for the lithium-ion secondary battery can also be one of conventionally known materials. The shape of the lithium-ion secondary battery can also be one of conventionally known shapes, such as coin-shaped, sheet-shaped (film-shaped), foldable, wound-type bottomed cylindrical, button-shaped, etc., which can be appropriately selected depending on the application.

[0055] <Method for producing sulfide-based solid electrolytes> The method for producing the sulfide-based solid electrolyte used in the lithium-ion secondary battery according to this embodiment involves a homogeneous intermediate compound before precipitation of argyrodite-type crystals. A homogeneous intermediate compound is not simply a mixture of multiple raw materials, but rather a compound formed when the raw materials react with each other, resulting in a structure that is, for example, amorphous or melted into a molten state.

[0056] To obtain a homogeneous amorphous intermediate compound, one method is to mix the raw materials using a media-less grinder such as a mixer mill or pin mill, and then mechanically mix them using a media grinder such as a planetary ball mill, bead mill, or attritor (registered trademark) to induce a mechanochemical reaction. Alternatively, the raw materials can be mixed using a mixer mill or pin mill and then dissolved; the dissolved state can also be called a homogeneous intermediate compound. Furthermore, the raw materials may be dissolved once in an organic solvent or the like.

[0057] The method for producing the sulfide-based solid electrolyte according to this embodiment is not particularly limited as long as an argyrodite-type crystal can be obtained via such a homogeneous intermediate compound, but two methods, i and ii described below, can be cited as examples.

[0058] (Manufacturing method i) Process i-1: A process of mixing raw materials to obtain a raw material mixture. Step i-2: A step of heating the above raw material mixture to obtain a molten product as an intermediate compound, and Step i-3: A step to cool the above molten material and precipitate argyrodite-type crystals. Includes.

[0059] (Manufacturing method ii) Step ii-1: A step of mixing raw materials to obtain an amorphous intermediate compound, and Step ii-2: A step of heating and calcining the above intermediate compound to precipitate argyrodite-type crystals. Includes. The intermediate compound obtained in step ii-1 above is an amorphous intermediate compound in which no peaks originating from the raw materials are observed in powder XRD measurement.

[0060] First, let's explain manufacturing method i. Step i-1 is the process of mixing raw materials to obtain a raw material mixture. To obtain argyrodite-type crystals, raw materials containing Li, P, S, and Ha are used. These can be conventionally known materials for obtaining argyrodite-type crystals containing Li, P, S, and Ha. For example, a mixture of a compound containing Li (lithium), a compound containing P (phosphorus), a compound containing S (sulfur), and a compound containing Ha (halogen) can be used. Furthermore, raw materials that serve as sources for elements other than S, Cl, and Br present in the free anion sites, as well as elements that substitute for P at the 4b site and S at the 16e site, are also mixed in.

[0061] Examples of lithium-containing compounds include lithium compounds such as lithium sulfide (Li2S), lithium oxide (Li2O), lithium carbonate (Li2CO3), lithium hydroxide (LiOH), and lithium sulfate (Li2SO4), as well as elemental lithium metal.

[0062] Examples of compounds containing phosphorus (P) include phosphorus sulfides such as diphosphorus trisulfide (P2S3) and diphosphorus pentasulfide (P2S5), phosphorus compounds such as lithium phosphate (LiPO3, Li4P2O7, Li3PO4) and sodium phosphate (NaPO3, Na4P2O7, Na3PO4), and elemental phosphorus. Examples of sulfur-containing compounds include lithium sulfide (Li2S), phosphorus sulfide (P2S3, P2S5), and hydrogen sulfide (H2S), and elemental sulfur can also be used. Among compounds containing ha, those containing chlorine (Cl) include, for example, lithium chloride (LiCl), phosphorus trichloride (PCl3), phosphorus pentachloride (PCl5), phosphorus tetrachloride (P2Cl4), phosphoryl chloride (POCl3), sulfur dichloride (SCl2), disulfur dichloride (S2Cl2), sodium chloride (NaCl), and boron trichloride (BCl3). Among compounds containing ha (Ha), examples of compounds containing br (bromine) include lithium bromide (LiBr), phosphorus tribromide (PBr3), phosphoryl chloride (POBr3), disulfur dibromide (S2Br2), sodium bromide (NaBr), and boron tribromide (BBr3). In particular, a combination of lithium sulfide, phosphorus sulfide, and at least one of lithium chloride and lithium bromide is preferred.

[0063] Examples of raw materials that serve as sources for elements other than S, Cl, and Br present in the free anion site, or for elements that substitute for P at the 4b site or S at the 16e site, include sulfides such as silicon dioxide (SiO2), tin oxide (SnO), silicon disulfide (SiS2), tin disulfide (SnS2), germanium disulfide (GeS2), vanadium(III) sulfide (V2S3), and zirconium disulfide (ZrS2); alkali metal oxides and alkali metal hydroxides such as selenium (Se), lithium oxide (Li2O), lithium hydroxide (LiOH), and sodium oxide (Na2O); alkali metal earth oxides; and fluorides such as LiF.

[0064] Depending on the raw materials used, some may be highly unstable in the atmosphere, reacting with water and decomposing, potentially generating hydrogen sulfide gas or causing oxidation. In such cases, mixing in an inert atmosphere is preferable. If no atmospherically unstable raw materials are used, mixing may be carried out in the atmosphere.

[0065] The raw materials can be mixed using methods such as a mixer mill, pin mill, powder agitator, or media-less mixing such as airflow mixing. The raw materials may be partially amorphous by mixing before step i-2.

[0066] Step i-2 is a step in which the obtained raw material mixture is heated to obtain a molten product as an intermediate compound. A molten state means that, in high-temperature X-ray diffraction measurements, no peaks originating from the raw materials are observed. In the case of argyrodite crystals, for example, with a Li-PS-Ha composition, the melt mixes well, indicating that it is a homogeneous molten compound distinct from the raw materials. A simple way to confirm whether a material is molten is to observe the state of the raw materials inside the furnace. If no unmelted material is observed, it is completely dissolved and can be considered an intermediate compound.

[0067] By going through an intermediate compound, it becomes easier to obtain argyrodite-type crystals that satisfy at least one of the following conditions: (A) the free anion site contains S, at least one of Cl and Br, and one or more elements other than those, and (B) a portion of the P at the 4b site and a portion of the S at the 16e site adjacent to the P at the 4b site are each substituted with other elements.

[0068] High-temperature X-ray diffraction measurements are performed after setting the measurement temperature, holding time, atmosphere, etc., to match the heating conditions used to obtain the molten material. Furthermore, by performing high-temperature X-ray diffraction measurements while varying the measurement temperature, it becomes possible to track changes in the crystalline state, such as phase transitions, and estimate the heating temperature at which an intermediate compound can be obtained.

[0069] The heating conditions required to obtain a molten product vary depending on the composition of the raw materials or raw material mixture used. The heating temperature should be above the temperature at which a homogeneous amorphous intermediate compound as described above is obtained. For example, 550°C or higher is preferred, 600°C or higher is more preferred, and 650°C or higher is even more preferred. Furthermore, from the viewpoint of suppressing compositional deviations due to volatilization of components, the heating temperature should be 950°C or lower, 900°C or lower is more preferred, and 850°C or lower is even more preferred. The temperature may also be changed in steps within the above temperature range. In addition, the higher the halogen content expressed as [Ha] / [P] (atomic ratio) in the composition, the lower the heating temperature can be.

[0070] The heating time should be at least the time required to obtain the homogeneous amorphous intermediate compound described above. This time varies depending on the scale, but for example, 2 minutes or more is preferred, 5 minutes or more is more preferred, and 10 minutes or more is even more preferred. Furthermore, from the viewpoint of productivity, the heating time should be 360 ​​minutes or less, more preferably 180 minutes or less, and even more preferably 120 minutes or less.

[0071] By stirring during heating and melting, the molten material, which will become an amorphous intermediate compound, can be made more homogeneous. Furthermore, the more homogeneous the mixing in step i-1, the shorter the heating time in the subsequent step i-2 can be.

[0072] The specific method of heating and melting is not particularly limited, but one example is to place the raw material in a heat-resistant container and heat it in a heating furnace. The heat-resistant container is not particularly limited, but examples include heat-resistant containers made of carbon, heat-resistant containers containing oxides such as quartz, quartz glass, borosilicate glass, aluminosilicate glass, alumina, zirconia, and mullite, heat-resistant containers containing nitrides such as silicon nitride and boron nitride, and heat-resistant containers containing carbides such as silicon carbide. Furthermore, these heat-resistant containers may be formed as a bulk from the above materials, or they may be containers in which layers of carbon, oxides, nitrides, carbides, etc., are formed.

[0073] The raw materials are preferably mixed in an inert atmosphere, and the molten product is preferably obtained by heating it in the same inert atmosphere. Examples of an inert atmosphere include an Ar atmosphere and a nitrogen atmosphere, but from the viewpoint of production costs, a nitrogen atmosphere is more preferable. The molten product may also be obtained by heating it in a hydrogen sulfide gas atmosphere, a sulfur gas atmosphere, a sulfur dioxide gas atmosphere, or a mixed gas atmosphere of these gases. Alternatively, the raw materials may be heated in a vacuum-sealed state.

[0074] When Ha in an argyrodite-type crystalline phase contains Br, Br has a larger ionic radius than Cl, making it difficult for it to enter the crystalline structure. Therefore, by adjusting the heating conditions to obtain the molten material, such as raising the heating temperature above the above range, increasing the heating time, or doing both, it becomes easier to obtain a homogeneous intermediate compound. However, in manufacturing method i, where the intermediate compound is a molten material, the intermediate compound tends to become homogeneous due to its high fluidity. Therefore, Br, which has a large ionic radius, can easily be incorporated into the crystal structure, and the above-mentioned adjustments are not necessarily required.

[0075] For amorphous intermediate compounds, it is acceptable if no peaks originating from the raw materials are observed in high-temperature X-ray diffraction measurements. However, from the viewpoint of homogeneity, it is preferable that the melt is homogeneous and not phase-separated. When the argyrodite-type crystal has a Li-PS-Ha composition, the melt mixes well without phase separation, but the presence or absence of phase separation can be confirmed visually. Alternatively, the presence or absence of phase separation can be confirmed optically by whether or not light is transmitted without scattering.

[0076] Step i-3 is a process of cooling the molten amorphous intermediate compound to precipitate argyrodite-type crystals. It should be noted that impurities originating from raw materials, etc., may be present in the argyrodite-type crystals, as long as they do not affect the lithium ion conductivity or elastic modulus of the solid electrolyte.

[0077] The cooling conditions required for crystal precipitation vary depending on the composition and the desired crystallization rate. The cooling rate is not particularly limited as long as argyrodite-type crystals precipitate, but from the viewpoint of productivity, it is preferably 5°C / min or more, more preferably 10°C / min or more, and even more preferably 30°C / min or more. Furthermore, from the viewpoint of increasing the crystallization rate, the cooling rate is preferably 2000°C / min or less, more preferably 1000°C / min or less, and even more preferably 300°C / min or less.

[0078] During cooling, extending the residence time between 200 and 450°C to allow for crystal growth or crystal structure reconstruction is more preferable from the viewpoint of improving lithium-ion conductivity. Alternatively, after cooling to room temperature, an additional heat treatment at a temperature of 200 to 550°C may be performed.

[0079] The cooling atmosphere should preferably be an inert atmosphere, similar to the atmosphere used during mixing the raw materials and heating to obtain the molten product. Furthermore, if the heating to obtain the molten product is performed in a vacuum-sealed container, the cooling may also be performed in the same vacuum-sealed container.

[0080] Thus, by employing the manufacturing method i according to this embodiment, the free anion site can contain S, at least one of Cl and Br, and one or more other elements. Furthermore, a portion of the P at the 4b site and a portion of the S at the 16e site adjacent to the P at the 4b site can be substituted with other elements. Moreover, the anion parameter value can be set to 0.36 or less. As a result, the deterioration of battery characteristics during repeated charging and discharging can be suppressed even without treating the surface of the positive electrode active material with a coating agent. The preferred embodiment of the obtained solid electrolyte is the same as the preferred embodiment described above for <sulfide-based solid electrolyte>.

[0081] Next, we will explain manufacturing method ii. Step ii-1 is a step in which raw materials are mixed to obtain a homogeneous amorphous intermediate compound.

[0082] To obtain argyrodite-type crystals, raw materials containing Li, P, S, and Ha are used, and these can be the same as those described in step i-1 of manufacturing method i, under the same conditions. In addition, raw materials that serve as sources for elements other than S, Cl, and Br present at the free anion sites, as well as elements that substitute for P at the 4b site and S at the 16e site, are also mixed in. These can be the same as those described in step i-1 of manufacturing method i and used under the same conditions.

[0083] By employing much stricter conditions than conventional methods in the mixing of these raw materials, amorphous intermediate compounds can be obtained. An amorphous (non-crystalline) intermediate compound means that, in X-ray diffraction measurements, no peaks originating from the raw materials are observed, which indicates that it is a homogeneous compound distinct from the mixture of raw materials.

[0084] By passing through an amorphous intermediate compound, it becomes easier to obtain argyrodite-type crystals that satisfy at least one of the following conditions: (A) the free anion site contains S, at least one of Cl and Br, and one or more elements other than those, and (B) a portion of the P at the 4b site and a portion of the S at the 16e site adjacent to the P at the 4b site are each substituted with other elements.

[0085] When using a mechanical milling method with a ball mill for mixing raw materials, examples include rotary ball mills that impart rotational motion to the container, vibrating ball mills that impart vibrational motion, planetary ball mills that impart both revolutionary and rotational motion, bead mills, and attritors (registered trademark). All of these can be applied if conditions are adopted that yield amorphous intermediate compounds. Among these, planetary ball mills and bead mills, which have higher mixing and grinding power, are preferred.

[0086] In the mechanical milling method using a ball mill, the higher the rotation speed, the longer the mixing time, and the smaller the ball particle size, the higher the mixing and grinding power. The rotational speed varies depending on the type of ball mill used and other conditions, but is preferably 200 rpm or higher, more preferably 300 rpm or higher, and even more preferably 400 rpm or higher. There is no particular upper limit to the rotational speed, but from the viewpoint of mechanical strength, it is preferably 1000 rpm or lower, and more preferably 800 rpm or lower. Alternatively, you can combine multiple rotation speeds, such as mixing at a low speed for a certain period of time first, and then increasing the rotation speed to a high speed for another period of time.

[0087] The mixing time varies depending on the type of ball mill used and other conditions, but is preferably 0.5 hours or more, more preferably 2 hours or more, and even more preferably 4 hours or more. There is no particular upper limit to the mixing time, but from the viewpoint of productivity, it is preferably 50 hours or less, and more preferably 20 hours or less.

[0088] The ball diameter varies depending on the type of ball mill used and other conditions, but is preferably 20 mm or less, more preferably 10 mm or less, and even more preferably 5 mm or less. The lower limit of the ball diameter is preferably 0.3 mm or more, and more preferably 1 mm or more, from the viewpoint of ease of handling. In addition, two or more types of balls with different diameters may be used in combination.

[0089] The amount of balls used varies depending on the type of ball mill used and other conditions, but from the viewpoint of mixing force and grinding force, it is preferable that the amount of balls be 20% or more of the total weight of the raw materials, more preferably 50% or more, even more preferably 100% or more, and also preferable that it be 1000% or less, more preferably 500% or less, and even more preferably 400% or less.

[0090] Ball milling can be used for dry mixing or wet mixing using a dispersion medium, but dry mixing is preferred from the viewpoint of efficiently transferring energy.

[0091] Through the mixing process described above, not only are the raw materials mixed, but the mixed powder becomes amorphous, resulting in a homogeneous amorphous intermediate compound. As mentioned earlier, the resulting amorphous intermediate compound means that no XRD peaks originating from the raw materials are observed. However, if the Raman spectrum shows that the peak at the raw material's position has completely disappeared and a peak has appeared at a different position, it can be considered that a more homogeneous intermediate has been obtained.

[0092] Powder X-ray diffraction measurements used to verify the obtained amorphous intermediate compounds may not show peaks even in crystalline form if the particle size of the sample powder is very small. Therefore, from the viewpoint of distinguishing between a mixture of raw materials with very small particle sizes and the amorphous intermediate compound, the particle size of the intermediate compound is preferably 30 nm or larger. Furthermore, from the viewpoint of handling the powder, the particle size of the intermediate compound is preferably 100 nm or larger, and more preferably 1 μm or larger. On the other hand, in order to increase the surface area of ​​the powder and efficiently advance the subsequent reaction, the particle size of the intermediate compound is preferably 50 μm or smaller, more preferably 10 μm or smaller, and even more preferably 5 μm or smaller. In this specification, the particle size of the intermediate compound refers to the primary particle size determined from the image obtained by scanning electron microscopy (SEM) observation. An SEM measurement is performed without exposure to air, with a magnification of 2000x and an acceleration voltage of 2kV. The particle sizes of 20 particles that appear within a suitable field of view are measured, and the average value is determined as the primary particle size.

[0093] Step ii-2 is a process in which the amorphous intermediate compound obtained in step ii-1 is heated and calcined to precipitate argyrodite-type crystals. As mentioned above, impurities originating from raw materials, etc., may be present in the argyrodite-type crystals, as long as they do not affect the lithium ion conductivity or elastic modulus of the solid electrolyte.

[0094] Heating is preferably carried out, for example, under an inert gas atmosphere, a hydrogen sulfide gas atmosphere, a sulfur gas atmosphere, or under a vacuum-sealed tube. From the viewpoint of promoting solid-phase reactions, i.e., crystallization, the heating temperature is preferably 350°C or higher, more preferably 400°C or higher, and even more preferably 450°C or higher. Furthermore, from the viewpoint of suppressing thermal decomposition, the heating temperature is preferably less than 600°C, and more preferably 575°C or lower.

[0095] From a similar viewpoint, the heating time is preferably 1 hour or more, more preferably 2 hours or more, and even more preferably 4 hours or more. Furthermore, the heating time is preferably 100 hours or less, more preferably 50 hours or less, and even more preferably 24 hours or less.

[0096] Thus, by employing manufacturing method ii according to this embodiment, the free anion sites can contain S, at least one of Cl and Br, and one or more other elements. Furthermore, a portion of the P at the 4b site and a portion of the S at the 16e site adjacent to the P at the 4b site can be substituted with other elements. Moreover, the anion parameter value can be set to 0.36 or less. As a result, the deterioration of battery characteristics during repeated charging and discharging can be suppressed even without treating the surface of the positive electrode active material with a coating agent. The preferred embodiment of the obtained solid electrolyte is the same as the preferred embodiment described above for <sulfide-based solid electrolyte>. [Examples]

[0097] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Examples 1-9 and 12-14 are examples, while Examples 10 and 11 are comparative examples.

[0098] (Example 1) Under a dry nitrogen atmosphere, lithium sulfide powder (Sigma, 99.98% purity), phosphorus pentasulfide powder (Sigma, 99% purity), lithium chloride powder (Sigma, 99.99% purity), lithium bromide powder (Sigma, 99.995% purity), and lithium oxide powder (Sigma, 97% purity) were weighed to match the composition ratios listed in "Composition" in Table 1. The raw material mixture was then mixed in the same atmosphere using a mixer (WARING, X-TREME (MX1100XTM)) on High mode for 1 minute to obtain the raw material mixture. The obtained raw material mixture was placed in a heat-resistant container and heated at 300°C for 1 hour under an atmosphere with a dew point of -60°C. The temperature was then increased and heated at 700°C for 0.5 hours to obtain a molten product. The mixture was then cooled to room temperature at a rate of 300°C / min to precipitate argyrodite-type crystals, thereby obtaining a sulfide-based solid electrolyte containing the desired argyrodite-type crystals. The upper panel of Figure 1 shows the results of XRD measurements (manufactured by Rigaku Corporation, SmartLab) of the obtained argyrodite-type crystals under the following conditions. From the obtained diffraction pattern, it was confirmed that the crystal is cubic. (conditions) The conditions for XRD measurement are as follows: Source: CuKα radiation (λ=1.5418Å), Tube voltage: 45kV, Tube current: 200mA, Scanning angle: 10~100°, Scanning speed: 5° / min, Step count: 0.01° / step.

[0099] (Examples 2-7, 9) Examples 2-7 and 9, containing argyrodite-type crystals, were obtained in the same manner as in Example 1, except that the composition ratio was changed to that listed in "Composition" in Table 1. For Examples 2, 3, and 9, which contain F in their composition, lithium fluoride powder (Sigma, 99.98% purity) was weighed and used in addition to the lithium sulfide powder, phosphorus pentasulfide powder, lithium chloride powder, lithium bromide powder, and lithium oxide powder mentioned above. Lithium bromide powder was not used in Example 7, which does not contain Br in its composition. Lithium oxide powder was not used in Example 9, which does not contain O in its composition. The results of XRD measurements (SmartLab, Rigaku Corporation) performed on the argyrodite-type crystal obtained in Example 2 under the same conditions as in Example 1 are shown in the lower part of Figure 1. From the obtained diffraction pattern, it was confirmed that the crystal is cubic.

[0100] (Example 8) A sulfide-based solid electrolyte containing argyrodite-type crystals, as in Example 8, was obtained in the same manner as in Example 1, except that the composition ratio was changed to that listed in "Composition" in Table 1. The obtained sulfide-based solid electrolyte was subjected to XRD measurement (SmartLab, manufactured by Rigaku Corporation) under the following conditions. (conditions) The conditions for XRD measurement are as follows: Source: CuKα radiation (λ=1.5418Å), Tube voltage: 45kV, Tube current: 200mA, Scanning angle: 10~100°, Scanning speed: 5° / min, Step count: 0.01° / step.

[0101] The obtained XRD pattern is shown in Figure 2, where peaks were observed at 2θ = 15.9°, 18.2°, 25.6°, 25.9°, 30.0°, 30.3°, 31.2°, and 31.8°. From the peak positions, it was determined that the crystal is a single-phase argyrodite type with a rhombohedral crystal system.

[0102] (Examples 10 and 11) Under a dry nitrogen atmosphere, lithium sulfide powder (Sigma, 99.98% purity), phosphorus pentasulfide powder (Sigma, 99% purity), and lithium chloride powder (Sigma, 99.99% purity) were weighed to match the composition ratios listed in "Composition" in Table 1. After mixing in a mixer under the same atmosphere, the mixture was further mixed using a planetary ball mill (Ito Seisakusho Co., Ltd., LP-M2) to obtain a raw material mixture. Mixing with the planetary ball mill was performed using balls with a particle size of 10 mm at 400 rpm for 20 hours. The obtained raw material mixture was vacuum-sealed in a heat-resistant container and heated and calcined at 450°C for 5 hours to precipitate argyrodite-type crystals, obtaining a sulfide-based solid electrolyte containing argyrodite-type crystals. For Example 11, in addition to the lithium sulfide powder, phosphorus pentasulfide powder, and lithium chloride powder mentioned above, lithium bromide powder (manufactured by Sigma, purity 99.995%) was weighed and used. The obtained argyrodite-type crystals were confirmed to be cubic based on XRD measurements (SmartLab, manufactured by Rigaku Corporation) under the following conditions. (conditions) The measurement conditions are as follows: Source: CuKα radiation (λ=1.5418Å), Tube voltage: 45kV, Tube current: 200mA, Scanning angle: 10~100°, Scanning speed: 5° / min, Step count: 0.01° / step.

[0103] (Example 12) Under a dry nitrogen atmosphere, lithium sulfide powder (Sigma, 99.98% purity), phosphorus pentasulfide powder (Sigma, 99% purity), lithium chloride powder (Sigma, 99.99% purity), lithium bromide powder (Sigma, 99.995% purity), and lithium oxide powder (Sigma, 97% purity) were weighed to match the composition ratios listed in "Composition" in Table 2. The raw material mixture was then obtained by mixing in the same atmosphere using a mixer (WARING, X-TREME (MX1100XTM)) on High mode for 1 minute. The obtained raw material mixture was placed in a heat-resistant container and heated at 300°C for 1 hour under a dew point of -60°C atmosphere, then the temperature was increased to 700°C for 0.5 hours to obtain a molten product. The mixture was then cooled to room temperature at a rate of 300°C / min. In addition to the dew point of -60°C mentioned above, the experiment was also conducted under an N2 atmosphere with an oxygen concentration of 100 ppm or less. Subsequently, mixing was carried out using a planetary ball mill with balls having a particle size of 10 mm at 400 rpm for 20 hours. The resulting mixture was then vacuum-sealed into a heat-resistant container and heated and calcined at 450°C for 5 hours to obtain a sulfide-based solid electrolyte containing argyrodite-type crystals.

[0104] (Examples 13 and 14) Except for changing the composition ratio to that listed in "Composition" in Table 2, sulfide-based solid electrolytes containing argyrodite-type crystals of Examples 13 and 14 were obtained in the same manner as in Example 12. For Example 13, which contains Si in its composition, silicon disulfide (manufactured by American Elements, purity 99.9%) was weighed and used in addition to the lithium sulfide powder, phosphorus pentasulfide powder, lithium chloride powder, lithium bromide powder, and lithium oxide powder mentioned above. For Example 14, which contains Sn in its composition, tin disulfide (manufactured by Kojun Chemical Laboratory, purity 3Nup) was weighed and used in addition to the lithium sulfide powder, phosphorus pentasulfide powder, lithium chloride powder, lithium bromide powder, and lithium oxide powder mentioned above.

[0105] [Evaluation: Anion Parameters] The anion parameters of the obtained sulfide-based solid electrolytes were determined based on the results of synchrotron XRD measurements using the following procedure. (Synchrotron XRD measurement) For Examples 1, 3, 5, and 11-14, synchrotron X-ray diffraction measurements were performed under the following conditions. Measurement method: Powder X-ray diffraction Light energy used for measurement: 17.71 keV Sample shape: 0.3 mm diameter capillary Measurement angle range: 2θ = 0.1 to 95° Step size (Δ2θ) = 0.010° Detectors: Debye-Scherrer camera and two-dimensional semiconductor detector

[0106] (Structural analysis) For examples where synchrotron X-ray diffraction measurements were performed, the crystal structure was refined using the Rietveld method with RIETAN-FP software. The structure with the lowest Rwp value was identified as the crystal structure for each example. The Rwp value is a reliability factor (R-weighted pattern) that is generally used as a guideline for fitting the structure refinement using Rietveld analysis across the entire analysis range. A lower Rwp value is better, and in this analysis, the lowest Rwp value was less than 10% in all cases. This allowed us to determine the crystal structure of argyrodite-type crystals in sulfide-based solid electrolytes.

[0107] (Anion parameters) From the proportion of free anions determined from the structural analysis results, the {(1 / χ)} of the argyrodite-type crystals in each example (S) )×[S 2- ]+(1 / χ (O) )×[O 2- ]+(1 / χ (Br) )×[Br - ]+(1 / χ (Cl) )×[Cl - ]+(1 / χ (F) )×[F - The anion parameters represented by ]} were calculated. Note that the content of each anion is [S 2- ]+[O 2- ]+[Br - ]+[Cl - ]+[F - This is a normalized value such that ]=1. The electronegativity of each element in the anion parameter is χ (S) =2.5, χ (O) =3.5, χ (Br) =2.8, χ (Cl) =3.0, χ (F) = 4.0. For Examples 2, 4, and 6-10, the values ​​were obtained by first confirming that they were single-phase from the XRD patterns, and then referring to the analysis results of Examples 1, 3, and 5 obtained above, using the following formula. [S 2- ] = {(Value of S in Table 1 or Table 2) - (Value of P in Table 1 or Table 2) × 4} / Normalized value [O 2- ] = (Value of O in Table 1 or Table 2) / Normalized value [Br - ] = (Br value from Table 1 or Table 2) / Normalized value [M - ] = (Value of Cl in Table 1 or Table 2) / Normalized value [F - ] = (Value of F in Table 1 or Table 2) / Normalized value The above normalized value refers to [S 2- ]+[O 2- ]+[Br - ]+[Cl - ]+[F - This is a value that is adjusted so that ] = 1. The results are shown in the "Anion Parameters" column of Tables 1 and 2, respectively.

[0108] (Lithium-ion conductivity) A sulfide-based solid electrolyte was ground in a mortar, and after removing coarse particles using a 100 μm mesh pass, 100 mg was measured out. Next, the lithium-ion conductivity of the sample was measured using an AC impedance measuring device (Bio-Logic Sciences Instruments, potentiostat / galvanostat VSP) while pressurizing a 10 mm diameter area at 380 MPa. The measurement conditions were: measurement frequency: 100Hz to 1MHz, measurement voltage: 100mV, and measurement temperature: 25℃. The results are shown in Tables 1 and 2 under "Li ion conductivity (mS / cm)".

[0109] (Battery characteristics) Under a dry nitrogen atmosphere, a sulfide-based solid electrolyte was dry-milled using a planetary ball mill (Ito Seisakusho Co., Ltd., model LP-M2) with alumina balls of 2 mm in size. The average particle size D was then measured by passing the mixture through a sieve with a mesh size of 43 μm. 50 This yielded a sulfide-based solid electrolyte powder with a particle size distribution of 3 μm. Note that average particle size D 50 The particle size distribution was measured using a Microtrac MT3300EXII laser diffraction particle size distribution analyzer, and the resulting volume-based particle size distribution chart was used for the measurement. As the positive electrode active material, layered rock salt type NMC811 powder (purchased from MTI Corporation, volume average particle size: 11.75 μm) was used. A positive electrode composite material was prepared by mixing 34 parts by mass of the sulfide-based solid electrolyte powder prepared above, 60 parts by mass of the positive electrode active material, and 6 parts by mass of a conductive additive (acetylene black, manufactured by Denka Co., Ltd., HS100). 80 mg of the sulfide-based solid electrolyte powder prepared as described above was placed in a 10 mm diameter plastic cylinder and molded under pressure to form a solid electrolyte layer. Next, 6 mg of the positive electrode composite material prepared as described above was placed in the same cylinder and molded under pressure again to form a positive electrode layer. Furthermore, indium foil and lithium foil were placed on the opposite side from the positive electrode composite material to form a negative electrode layer. In this way, an all-solid-state lithium-ion secondary battery was prepared, and a charge-discharge test was performed at a confinement pressure of 10 kN.

[0110] The charge-discharge test was conducted under the following conditions: measurement temperature: 25°C, charging current density: 0.05C, discharging current density: 0.05C, charge-discharge potential range: 1.9~3.7V. A constant current charge-discharge test was performed for 5 cycles. The charge-discharge curve for Example 1 is shown in Figure 3. Based on the results of charge-discharge tests, the initial characteristics and capacity retention rate (%) were determined using the following formulas, and the battery characteristics of the all-solid-state lithium-ion secondary battery were evaluated. Initial characteristics = (Discharge capacity in the first cycle / Charge capacity in the first cycle) Capacity retention rate (%) = (Discharge capacity at 5th cycle / Discharge capacity at 1st cycle) × 100

[0111] The battery characteristics are shown in Tables 1 and 2 under the headings "Initial Characteristics," "Capacity Retention Rate (%)," and "Judgment," respectively. The criteria for judgment are as follows. ◎: Meets both the initial characteristic requirement of 0.65 or higher and the capacity retention rate requirement of 88% or higher. ○: Satisfies both initial performance of over 0.60 and capacity retention rate of over 86%, and also satisfies either initial performance of less than 0.65 or capacity retention rate of less than 88%. ×: Satisfies at least one of the following conditions: initial characteristic of 0.60 or less, and capacity retention rate of 86% or less.

[0112] (average bond distance) For Examples 12-14, PS4 3- The average binding distance between the 4b site and the adjacent 16e site in the structure was determined by synchrotron XRD measurements and Rietveld analysis. The results are shown in Table 2 under "Average Binding Distance".

[0113] [Table 1]

[0114] [Table 2]

[0115] Examples 1, 4-8, and 8 all contain argyrodite crystals with free anion sites containing sulfur (S) and oxygen (O) as an element other than at least one of Cl and Br. Example 9 also contains fluorine (F) as another element, while Examples 2 and 3 each contain both O and F. Furthermore, the anion parameter is 0.36 or less in all cases. These all resulted in good battery performance and suppressed degradation of battery characteristics after repeated charge-discharge cycles. On the other hand, in Examples 10 and 11, the other elements mentioned above were absent, and the degradation of battery characteristics after repeated charging and discharging was significant.

[0116] Furthermore, while the effects of the present invention are demonstrated in Example 12, where O is present as another element, in Examples 13 and 14, where in addition to O, a portion of the P at the 4b site is replaced with Si or Sn, the degradation of battery characteristics during repeated charging and discharging is suppressed more significantly, even though the anion parameter values ​​are the same.

[0117] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on Japanese Patent Application No. 2021-162230, filed on 30 September 2021, the contents of which are incorporated herein by reference.

Claims

1. A sulfide-based solid electrolyte used in lithium-ion secondary batteries, It contains argyrodite-type crystals containing Li, P, S, and Ha, The Ha is at least one element selected from the group consisting of F, Cl, Br, and I, which includes at least one of Cl and Br. The ratio of elemental content in the aforementioned argyrodite-type crystal satisfies the following relationships when the composition is expressed as Li α PS β Ha γ: 5 < α < 7, 3 < β < 6, and 0 < γ < 2.

5. The argyrodite-type crystal satisfies at least one of the following: (A) the free anion site contains S, at least one of Cl and Br, and one or more elements other than those, and (B) a portion of the P at the 4b site and a portion of the S at the 16e site adjacent to the P at the 4b site are each substituted with other elements. The said al-djurite type crystal has the content of the free anions [S ２－ , [O ２－ , [Br － , [Cl － and [F － existing in the said free anion sites, and their electronegativities χ （Ｓ） , χ （Ｏ） , χ （Ｂｒ） , χ （Ｃｌ） and χ （Ｆ） are such that {(1 / χ （Ｓ） )×[S ２－ +(1 / χ （Ｏ） )×[O ２－ +(1 / χ （Ｂｒ） )×[Br － +(1 / χ （Ｃｌ） )×[Cl － +(1 / χ （Ｆ） )×[F －} ≤ 0.36 and [S ２－ ]+[O ２－ ]+[Br － ]+[Cl － ]+[F － ]=1 A sulfide-based solid electrolyte that satisfies the following relationship.

2. The sulfide-based solid electrolyte according to claim 1, wherein the argyrodite-type crystal further comprises at least one of O and F.

3. Content of the aforementioned free anions [S ２－ ] [O ２－ ] [Br － ] [Cl － ] and [F － ], and their electronegativity χ （Ｓ） , χ （Ｏ） , χ （Ｂｒ） , χ （Ｃｌ） and χ （Ｆ） but, {(1 / x) （Ｓ） )×[S ２－ ]+(1 / x （Ｏ） )×[O ２－ ]+(1 / x （Ｂｒ） )×[Br － ]+(1 / x （Ｃｌ） )×[Cl] － ]+(1 / x （Ｆ） )×[F － ]}≦0.34 A sulfide-based solid electrolyte according to claim 1 or 2, satisfying the relationship.

4. The sulfide-based solid electrolyte according to claim 1 or 2, wherein the argyrodite-type crystal satisfies (B) above, and the average bond distance between the 4b site and the adjacent 16e site is 2.07 Å or more.

5. The argyrodite-type crystal satisfies (B) above, and the element M that is substituted for part of the P at the 4b site is PS ４ ３－ MS with a larger ionic radius ４ A sulfide-based solid electrolyte according to claim 1 or 2, which forms a tetrahedral structure.

6. Said MS ４ The sulfide-based solid electrolyte according to claim 5, wherein the element M in the tetrahedral structure is at least one of Si and Sn.

7. The argyrodite-type crystal satisfies (A) above, and the content of the free anions is 0 < {[Br － ] / ([F － ] + [Cl － ] + [Br － A sulfide-based solid electrolyte according to claim 1 or 2, satisfying the relationship}<2.

8. The sulfide-based solid electrolyte according to claim 1 or 2, wherein the electromotive force of the lithium-ion secondary battery is 4.3V or more.

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