JP6950916B2 - Lithium tin sulfide - Google Patents

Description

The present invention relates to lithium tin sulfide.

Lithium-ion secondary batteries are attracting attention as high-energy density batteries, and their applications are expanding not only to portable devices (small consumer applications) but also to stationary applications such as in-vehicle devices and social infrastructure. One of the requirements for these large lithium-ion secondary batteries is the improvement of safety. An organic electrolytic solution is used for a normal lithium-ion secondary battery (hereinafter abbreviated as liquid system), and a flammable low-boiling solvent (carbonate) that falls under the Fire Service Act Dangerous Goods Class 4 Second Petroleum as a viscosity reducing agent. Since it contains a large amount of dimethyl, diethyl carbonate, ethyl methyl carbonate, etc.), there is a concern that the battery may ignite or smoke. Although corporate efforts are being made to improve safety, if the components can be changed, that is, the electrolyte is changed to a solid electrolyte with lower flammability, and the material composition can be dramatically improved. , Cost reduction for safety consideration is achieved, and it is extremely useful in industry.

There are two types of solid electrolytes, polymer-based and inorganic-based. At present, polymer-based electrolytes have low ionic conductivity at room temperature or lower, and sufficient battery operation close to that of liquid-based electrolytes cannot be expected unless the temperature is 60 ° C or higher. On the other hand, there are oxide type and sulfide type in the inorganic type, and although the oxide type has high ionic conductivity, it has a problem that it is difficult to construct a battery only by pressing because it has low moldability and is brittle.

As such a solid electrolyte, various compounds have been found, but it cannot be said that it is sufficient yet, and other solid electrolytes are desired.

T. Kaib et al., J. Mater. Chem. A, 24 (11) 2211-2219 (2012).

The present invention has been made in view of the above-mentioned current state of the prior art, and a main object thereof is to provide a compound having high ionic conductivity and which can be used as a solid electrolyte for a lithium ion secondary battery. Is.

The present inventors have carried out diligent research to achieve the above-mentioned object. As a result, it was found that a sulfide having a specific composition and a specific X-ray diffraction pattern has an unprecedented crystal structure and is useful as a solid electrolyte. The present invention has been further studied and completed based on such findings. That is, the present invention includes the following configurations.
Item 1. Contains lithium, tin and sulfur as constituent elements
The composition ratio (Li / Sn) of the lithium and the tin is 3.0 to 7.0 in terms of molar ratio.
The composition ratio (S / Sn) of the sulfur and the tin is 3.0 to 5.5 in terms of molar ratio.
Crystalline lithium tin having the strongest peak or the second strongest peak at least at 25.5 ° within the allowable range of ± 1.0 ° within the diffraction angle 2θ = 10 ° to 80 ° in the X-ray diffraction pattern by CuKα rays. Sulfide.
Item 2. A crystal consisting of lithium tin sulfide having a composition ratio of lithium and tin (Li / Sn) of 3.0 to 5.0 in molar ratio and a composition ratio of sulfur and tin (S / Sn) of 3.0 to 5.0 in molar ratio. Item 2. The crystalline lithium tin sulfide having at least a phase.
Item 3. Item 2. The crystalline lithium tin sulfide according to Item 2, which has a crystal phase composed of lithium sulfide having a composition ratio (Li / S) of lithium and sulfur of 1.0 to 3.0 in molar ratio.
Item 4. Item 2. The crystalline lithium tin sulfide according to any one of Items 1 to 3, wherein the half width of the peak at 2θ = 25.5 ° is 0.20 to 1.05 °.
Item 5. Item 1 which has peaks at 27.9 °, 38.0 °, 45.0 ° and 50.1 ° within the allowable range of ± 1.0 ° within the range of diffraction angle 2θ = 10 ° to 80 ° in the X-ray diffraction diagram by CuKα rays. 4. The crystalline lithium tin sulfide according to any one of 4 to 4.
Item 6. Item 2. The crystalline lithium tin sulfide according to any one of Items 1 to 5, which has an average particle size of 1 to 30 μm.
Item 7. Item 2. The method for producing a crystalline lithium tin sulfide according to any one of Items 1 to 6.
A manufacturing method comprising a step of using lithium sulfide and tin sulfide as raw materials and subjecting them to a mechanical milling treatment.
Item 8. Item 7. The production method according to Item 7, wherein the amount of lithium sulfide used is 1.5 to 3.5 mol with respect to 1 mol of tin sulfide.
Item 9. Item 6. A solid electrolyte for a lithium ion secondary battery made of the crystalline lithium tin sulfide according to any one of Items 1 to 6.
Item 10. A lithium ion secondary battery using the crystalline lithium tin sulfide according to any one of Items 1 to 6 or the solid electrolyte for a lithium ion secondary battery according to Item 9.

The lithium tin sulfide of the present invention has high ionic conductivity and is useful as a solid electrolyte for a lithium ion secondary battery.

The X-ray diffraction pattern of _{the Li 4} SnS ₄ powder obtained in Examples 1 to 3 is shown. It is shown with the peaks of the raw materials Li ₂ S powder and Sn S _{2 powder.} In the vicinity of 2θ = 10 to 25 °, it is possible that the influence of the polymer non-exposure folder used to prevent atmospheric exposure has also been confirmed. The X-ray diffraction pattern using the high-energy X-ray of _{the Li 4} SnS ₄ powder obtained in Example 1 is shown. The peak of Kapton (registered trademark) used to prevent atmospheric exposure has also been detected. It is a two-body distribution function g (r) obtained by X-ray diffraction measurement using high-energy X-rays of _{the Li 4} SnS ₄ powder obtained in Example 1. It is a scanning electron microscope (SEM) photograph of _{the Li 4} SnS ₄ powder obtained in Example 1. The X-ray diffraction pattern of _{the Li 6} SnS ₅ powder obtained in Examples 4 to 6 is shown. It is shown together with the peaks of the sample of Example 1, the raw materials Li ₂ S powder and Sn S _{2 powder.} In the vicinity of 2θ = 10 to 25 °, it is possible that the influence of the polymer non-exposure folder used to prevent atmospheric exposure has also been confirmed.

1. 1. Crystalline Lithium Tin Sulfur The crystalline lithium tin sulfide of the present invention contains lithium, tin and sulfur as constituent elements, and the composition ratio (Li / Sn) of the lithium to the tin is 3.0 to 7.0 in terms of molar ratio. Yes, the composition ratio (S / Sn) of the sulfur and the tin is 3.0 to 5.5 in terms of molar ratio, and ± 1.0 within the range of the diffraction angle 2θ = 10 ° to 80 ° in the X-ray diffraction diagram with CuKα rays. A sulfide having the strongest peak or the second strongest peak at least at 25.5 °, within the tolerance of ° (especially ± 0.5 °). The crystalline lithium tin sulfide of the present invention having the above-mentioned characteristics is a compound having a crystal structure not previously reported, and high ionic conductivity can be obtained. In the present specification, the "strongest peak" means the peak having the highest intensity, and the "second strongest peak" means the peak having the second strongest intensity.

In the present invention, the X-ray diffraction spectrum is obtained by the powder X-ray diffraction measurement method. For example, the following measurement conditions:
X-ray source: CuKα 40kV-40mA
Measurement conditions: 2θ = 10 to 80 °, 0.01 ° step, scanning speed 0.5 seconds / step.

The crystalline lithium tin sulfide of the present invention has a diffraction peak at the above-mentioned 2θ position, but for a sulfide having good crystallinity, the diffraction angle is within the range of 2θ = 10 to 80 ° depending on the degree of crystallinity. In the allowable range of ± 1.0 °, diffraction peaks are also observed at at least one location (particularly all) of 27.9 °, 38.0 °, 45.0 ° and 50.1 °. For sulfides with even better crystallinity, depending on the degree of crystallinity, the allowable range of ± 1.0 ° (especially ± 0.5 °) within the range of diffraction angle 2θ = 10 to 80 ° is 27.0 °. Diffraction peaks are also observed at least at one of 30.2 °, 31.3 °, 52.8 °, 53.8 °, 55.6 °, 60.4 °, 65.3 °, 69.5 ° and 72.0 °.

The crystalline lithium tin sulfide of the present invention has a composition ratio (Li / Sn) of lithium and tin of 3.0 to 5.0 (particularly 3.5 to 4.5) in terms of molar ratio, and a composition ratio of sulfur and tin (S / Sn). ) Has at least a crystal phase composed of lithium tin sulfide having a molar ratio of 3.0 to 5.0 (particularly 3.5 to 4.5). The crystal phase composed of this lithium tin sulfide has a strong peak at the above-mentioned position of 2θ = 25.5 °, and the crystalline lithium tin sulfide of the present invention is a simple crystal phase composed of the above-mentioned lithium tin sulfide. In the case of a phase, the peak of 2θ = 25.5 ° is the strongest peak.

On the other hand, in the composition of the crystalline lithium tin sulfide of the present invention, the composition ratio of lithium and tin (Li / Sn) is 3.0 to 5.0 in terms of molar ratio, and the composition ratio of sulfur and tin (S / Sn) is Other than the composition having a molar ratio of 3.0 to 5.0 (for example, Li / Sn is 5.0 to 7.0 (especially 5.0 to 6.3) and S / Sn is 4.0 to 5.5 (especially 4.0 to 5.3)). In some cases, etc.), not only the above-mentioned crystal phase composed of lithium tin sulfide, but also lithium sulfide having a composition ratio (Li / S) of lithium and sulfur of 1.0 to 3.0 (particularly 1.5 to 2.5) in molar ratio. It also has a crystalline phase consisting of material. In this case, the peak at 2θ = 27.0 ° may be the strongest peak, and the peak at 2θ = 25.5 ° may be the second strongest peak.

By adopting such a crystal structure, it is possible to further increase the ionic conductivity.

Further, in the crystalline lithium tin sulfide of the present invention, the half width of the peak at 2θ = 25.5 ° is preferably 0.20 to 1.05 °, more preferably 0.25 to 0.85 °. Since the full width at half maximum of the strongest peak exists in this range, highly conductive crystallinity and low crystalline component that assists ion conduction between particles by further reducing grain boundary resistance coexist. It is easy to construct a densely molded body because it has both conductivity and moldability.

Crystalline lithium tin sulfides present invention are those having a diffraction peak described above for the ratio of the respective elements, in particular, the composition formula: when expressed in Li _n1 SnS _m1, n1 and m1, respectively It is considered that the crystal structure of the lithium tin sulfide of the present invention is most stabilized when n1 = 4 and m1 = 4. Further, when n1 and m1 satisfy the relationship of n1≈2 (m1) -4 and n1> 4, for example, when n1 = 6 and m1 = 5, a lithium tin sulfide crystal having the above crystal structure It is considered to have a two-phase structure consisting of a phase and a lithium sulfide crystal phase. From this point of view, the composition ratio Li / Sn of lithium Li and tin Sn is 3.0 to 7.0, preferably 3.5 to 6.5 in terms of molar ratio, and the composition ratio S / Sn of sulfur S and tin Sn is molar. The ratio is 3.0 to 5.5, preferably 3.5 to 5.3. Specifically, when Li / Sn is 3.5 to 4.5 and S / Sn is 3.5 to 4.5, the crystal structure composed of the above-mentioned lithium tin sulfide crystal phase can be stabilized. When Li / Sn is 4.0 to 7.0 and S / Sn is 4.0 to 5.5, a two-phase structure of a lithium tin sulfide crystal phase having the above-mentioned crystal structure and a lithium sulfide crystal phase can be obtained.

Specific examples of the crystalline lithium tin sulfide that satisfies such conditions include the general formula (1):
Li _n1 SnS _m1 (1)
[In the equation, n1 and m1 satisfy 3.0 ≦ n1 ≦ 7.0 (preferably 3.5 ≦ n1 ≦ 6.5); 3.0 ≦ m1 ≦ 5.5 (preferably 3.5 ≦ m1 ≦ 5.3). ]
Examples thereof include crystalline lithium tin sulfide represented by. Among them, the general formula (1A):
Li _n1 SnS _m1 (1A)
[In the equation, n1 and m1 satisfy 3.0 ≦ n1 ≦ 5.0 (preferably 3.5 ≦ n1 ≦ 4.5); 3.0 ≦ m1 ≦ 5.0 (preferably 3.5 ≦ m1 ≦ 4.5). ]
The crystalline lithium tin sulfide represented by can have a crystal structure with few heterogeneous phases, and has a general formula (1B):
Li _n1 SnS _m1 (1B)
[In the equation, n1 and m1 satisfy 4.0 ≤ n1 ≤ 7.0 (preferably 4.0 ≤ n1 ≤ 6.5); 4.0 ≤ m1 ≤ 5.5 (preferably 4.0 ≤ m1 ≤ 5.3). ]
The crystalline lithium tin sulfide represented by is preferable because it can have a two-phase structure of a lithium tin sulfide crystal phase having the above-mentioned crystal structure and a lithium sulfide crystal phase.

In the case of adopting a two-phase structure, the abundance ratio of each phase is not particularly limited, but since the lithium tin sulfide crystal phase has excellent ionic conductivity, the abundance of the lithium tin sulfide crystal phase is large. Is preferable. From this point of view, the lithium tin sulfide crystal phase is preferably 10 to 95 mol% (particularly 30 to 90 mol%), and the lithium sulfide crystal phase is preferably 5 to 90 mol% (particularly 10 to 70 mol%). preferable.

The composition of the crystalline lithium tin sulfide whose crystal structure is most stabilized is Li ₄ SnS ₄ , and in the Li ₆ SnS ₅ composition, the Li ₄ SnS ₄ crystal phase and the Li ₂ S crystal phase are two. It can take a phase structure.

_{Further, regarding the relationship between n1 and m1} in the above general formula (1): Li _n1 SnS m1, it is preferable when n1 = m1≈4 because the crystallinity can be easily increased. Even if n1 ≠ m1, if n1 ≈ 2 (m1) -4, the cations and anions in the crystal phase containing tin can be balanced by adopting the above-mentioned two-phase structure. , Lithium tin sulfide having a crystal structure composed of the above-mentioned lithium tin sulfide crystal phase can be easily obtained.

The average particle size of the crystalline lithium tin sulfide of the present invention is preferably 1 to 30 μm, more preferably 2 to 20 μm. The average particle size of the crystalline lithium tin sulfide of the present invention is measured by electron microscope (SEM) observation.

Further, the crystalline lithium tin sulfide of the present invention preferably has an ionic conductivity of 1 × 10 ^-6 to 1 × 10 ^-3 Scm ^-1 when pelletized, and 1 × 10 ^-5 to 1 More preferably, it is × 10 ^-3 Scm ^-1. In particular, it is preferable that the ionic conductivity is 10 times or more higher than the electron conductivity calculated from the electron resistance value when the crystalline lithium tin sulfide of the present invention is pelletized. The electron conductivity of the crystalline lithium tin sulfide of the present invention is measured by the DC polarization method. The ionic conductivity is measured by an AC impedance method using a blocking electrode.

The crystalline lithium tin sulfide of the present invention satisfies the above conditions, but may contain other impurities as long as it does not impair the performance of the crystalline lithium tin sulfide. Examples of such impurities include transition metals and metals such as main group elements that may be mixed in the raw material; carbon, oxygen and the like that may be mixed in the raw material and during production. Further, residues of raw materials (lithium sulfide (Li ₂ S), tin sulfide (SnS ₂ ), etc.) and products other than the object of the present invention may be contained as impurities. The amount of these impurities may be a range that does not impair the performance of the above-mentioned crystalline lithium tin sulfide, and is usually the total amount of lithium, tin and sulfur in the crystalline lithium tin sulfide that satisfies the above conditions. With respect to 100 parts by mass, 10 parts by mass or less is preferable, 5 parts by mass or less is more preferable, and 3 parts by mass or less is further preferable.

When these impurities are present, in addition to the peaks in the above-mentioned X-ray diffraction diagram, diffraction peaks may be present depending on the impurities.

2. Method for Producing Crystalline Lithium Tin Sulfide The crystalline lithium tin sulfide of the present invention can be obtained, for example, by using lithium sulfide (Li ₂ S) and tin sulfide (SnS ₂ ) as raw materials and subjecting them to a mechanical milling treatment. be able to.

The mechanical milling treatment is a method of grinding and mixing raw materials while applying mechanical energy. According to this method, lithium sulfide and sulfurization are mixed by applying mechanical impact and friction to the raw materials. The tin violently contacts and becomes finer, causing a reaction of the raw materials. That is, at this time, mixing, pulverization and reaction occur at the same time. Therefore, it is possible to react the raw material more reliably without heating the raw material to a high temperature. By using the mechanical milling treatment, a metastable crystal structure that cannot be obtained by ordinary heat treatment may be obtained.

Specifically, as the mechanical milling process, mixed pulverization can be performed using a mechanical pulverizer such as a ball mill, a bead mill, a rod mill, a vibration mill, a disc mill, a hammer mill, or a jet mill.

The lithium sulfide (Li ₂ S) used as a raw material is not particularly limited, and commercially available lithium sulfide can be used. In particular, it is preferable to use a high-purity product. Further, since lithium sulfide is mixed and pulverized by a mechanical milling treatment, the particle size of lithium sulfide used is not limited, and commercially available powdered tin sulfide can be usually used.

The tin sulfide (SnS ₂ ) used as a raw material is also not particularly limited, and any commercially available tin sulfide can be used. In particular, it is preferable to use a high-purity product. Further, since tin sulfide is mixed and pulverized by a mechanical milling treatment, the particle size of tin sulfide used is not limited, and commercially available powdered tin sulfide can be usually used.

The mixing ratio of these raw materials can be the same as the element ratio of lithium, tin and sulfur in the target lithium tin sulfide. Specifically, the amount of lithium sulfide used is preferably 1.5 to 3.5 mol, more preferably 1.8 to 3.2 mol, with respect to 1 mol of tin sulfide. When the amount of lithium sulfide used is 1.75 to 2.25 mol per 1 mol of tin sulfide, a single phase of a lithium tin sulfide crystal phase having a particularly stable crystal structure can be easily produced, and the amount of lithium sulfide used can be increased. When the amount is 2.0 to 3.5 mol per 1 mol of tin sulfide, a two-phase structure of a lithium tin sulfide crystal phase and a lithium sulfide crystal phase having a particularly stable crystal structure can be easily produced.

The temperature at which the mechanical milling treatment is performed is not particularly limited, but from the viewpoint of making it difficult for sulfur to volatilize and forming the previously reported crystal phase, it is mechanical at 300 ° C or lower, preferably -10 to 200 ° C. It is preferable to perform milling treatment.

The time of the mechanical milling treatment is not particularly limited, and the mechanical milling treatment can be performed for an arbitrary time until the desired crystalline lithium tin sulfide is precipitated.

For example, the mechanical milling treatment can be performed with an energy amount of about 0.1 to 100 kWh / raw material mixture of about 1 kg within a treatment time range of about 0.1 to 100 hours. It should be noted that this mechanical milling process can be performed in a plurality of times with a pause in the middle, if necessary.

By the mechanical milling treatment described above, the desired crystalline lithium tin sulfide can be obtained as a fine powder.

3. 3. Applications of Crystalline Lithium-tin Sulfuric Acid The crystalline lithium-tin sulfide of the present invention described above is useful as an ionic conductor (particularly a solid electrolyte) because of its excellent ionic conductivity as described above, and is particularly useful as a lithium ion nitrous. It is preferable to use it as a solid electrolyte for a secondary battery.

When the crystalline lithium tin sulfide of the present invention is used as a solid electrolyte for a lithium ion secondary battery, the structure of the lithium ion secondary battery of the present invention (particularly an all-solid lithium ion secondary battery) is the crystal of the present invention. It can be the same as that of a known lithium ion secondary battery except that the sex lithium tin sulfide is used as a solid electrolyte.

For example, as the positive electrode, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium iron phosphate (LiFePO ₄ ), vanadium oxide-based material, sulfur-based material, etc. A positive electrode mixture containing the positive electrode active material and, if necessary, a conductive agent and a binder can be supported on a positive electrode current collector such as Al, Ni, stainless steel, or carbon cloth using the known positive electrode active material. As the conductive agent, for example, a carbon material such as graphite, coke, carbon black, or acicular carbon can be used. Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyimide (PI), polyamide, polyamideimide, polyacrylic, styrene butadiene rubber (SBR), and styrene-ethylene-butylene-styrene. Materials such as copolymer (SEBS) and carboxymethyl cellulose (CMC) can be used alone, or two or more of them can be used in combination.

Further, as the negative electrode, a known negative electrode active material such as metallic lithium, carbon-based material (activated carbon, graphite, etc.), silicon, silicon oxide, Si-SiO-based material, lithium titanium oxide, etc. is used, and this negative electrode active material is required. A positive electrode mixture containing a conductive agent and a binder can be supported on a negative electrode current collector such as Al, Ni, stainless steel, or carbon cloth. As the conductive agent, for example, a carbon material such as graphite, coke, carbon black, or acicular carbon can be used. Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyimide (PI), polyamide, polyamideimide, polyacrylic, styrene butadiene rubber (SBR), and styrene-ethylene-butylene-styrene. Materials such as copolymer (SEBS) and carboxymethyl cellulose (CMC) can be used alone, or two or more of them can be used in combination.

Further, as the electrolyte layer, the crystalline lithium tin sulfide of the present invention can be formed into a layer by a conventional method using a known binder, if necessary, and used as the electrolyte layer. Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyimide (PI), polyamide, polyamideimide, polyacrylic, styrene butadiene rubber (SBR), and styrene-ethylene-butylene-styrene copolymer weight. Materials such as coalescence (SEBS) and carboxymethyl cellulose (CMC) can be used alone, or two or more of them can be used in combination.

As the separator, for example, it is made of a material such as a polyolefin resin such as polyethylene or polypropylene, a fluororesin, nylon, an aromatic aramid, or an inorganic glass, and a material in the form of a porous film, a non-woven fabric, a woven fabric, or the like can be used.

In addition, other known battery components can be used to assemble lithium-ion secondary batteries according to conventional methods. In the present invention, the "lithium ion secondary battery" is a concept that also includes a "lithium secondary battery" that uses metallic lithium as a negative electrode material.

The shape of the lithium ion secondary battery is not particularly limited, and may be any of a cylindrical type, a square type, and the like.

Hereinafter, the present invention will be described in more detail with reference to examples. However, it goes without saying that the present invention is not limited to the following examples.

[Example 1: Synthesis of _{Li 4} SnS _{4 powder]}
In a glove box in an argon atmosphere (non-exposed to the atmosphere), commercially available lithium sulfide (Li ₂ S) powder and tin sulfide (SnS ₂ ) powder are mixed in a molar ratio of 2: 1 (mass ratio 0.5017: 0.9983). Weighed and mixed, and then mechanical milling at 510 rpm for 50 hours with a ball mill device (Fritsch P-7, Classic Line) using a 45 mL zirconia container containing about 500 (90 g) zirconia balls with a diameter of 4 mm. To _{obtain Li 4} SnS ₄ powder. In the mechanical milling process, the milling process for 1 hour was performed 50 times in total in the overpot for atmosphere control with a pause of 15 minutes. The obtained _{powder having a Li 4} SnS ₄ composition is referred to as Li ₄ SnS ₄ powder for convenience. The same applies to Examples 2 and 3. The powder density of the obtained Li ₄ SnS ₄ powder was measured with a dry powder density meter (Accupic II 1340-1CC manufactured by Makelomericix) and found to be 2.61 g cm ^-3 .

[Example 2: Synthesis of _{Li 4} SnS _{4 powder]}
In a glove box in an argon atmosphere (non-exposed to the atmosphere), commercially available lithium sulfide (Li ₂ S) powder and tin sulfide (SnS ₂ ) powder are mixed in a molar ratio of 2: 1 (mass ratio 0.5017: 0.9983). Weighed and mixed, and then mechanical milling at 510 rpm for 40 hours using a 45 mL zirconia container containing about 500 (90 g) zirconia balls with a diameter of 4 mm using a ball mill device (Fritsch P-7, Classic Line). To _{obtain Li 4} SnS ₄ powder. The mechanical milling process was carried out in an atmosphere control overpot for a total of 40 times with a 1-hour milling process and a 15-minute pause.

[Example 3: Synthesis of _{Li 4} SnS _{4 powder]}
In a glove box in an argon atmosphere (non-exposed to the atmosphere), commercially available lithium sulfide (Li ₂ S) powder and tin sulfide (SnS ₂ ) powder are mixed in a molar ratio of 2: 1 (mass ratio 0.5017: 0.9983). Weighed and mixed, and then mechanical milling at 510 rpm for 20 hours using a 45 mL zirconia container containing about 500 (90 g) zirconia balls with a diameter of 4 mm using a ball mill device (Fritsch P-7, Classic Line). To _{obtain Li 4} SnS ₄ powder. The mechanical milling treatment was carried out in an atmosphere control overpot for a total of 20 times with a 1-hour milling treatment and a 15-minute pause.

[Test Example 1: Measurement of conductivity of solid electrolyte pellets (Part 1)]
_{80 mg of Li 4} SnS ₄ powder obtained in Example 1 was homogeneously filled in a molding machine having a diameter of 10 mm, and uniaxially molded at 330 MPa for 1 minute to prepare pellets having a thickness of 0.501 mm and a diameter of 10.02 mmΦ. When the density of this pellet was calculated from the shape, it was 2.03 gcm ^-3 (relative density 77.8%). The resistance of this pellet at 25 ° C was measured by the AC impedance method and found to be 2,001 Ω, and the ionic conductivity σ of this pellet at 25 ° C was 3.17 × 10 ^-5 Scm ^-1 . It was found that the electron resistance value measured by the DC polarization method was at least 1 MΩ or more, the electron conductivity was 6 × 10 ^-8 Scm ^-1 or less, and the electron conductivity was 1/500 or less of the ionic conductivity. rice field.

[Test Example 2: X-ray diffraction (1)]
X-ray diffraction (XRD) using CuKα rays was measured for the powders obtained in Examples 1 to 3. The results are shown in FIG. For reference, FIG. 1 also shows the peaks of _{lithium sulfide (Li 2} S) and tin sulfide (SnS _{2) used as raw materials.} In addition, in the XRD measurement, in order to avoid exposure to the atmosphere of the prepared sample, a polymer-made air-unexposed folder was used, so it is possible that some effects of the polymer were confirmed around 2θ = 10 to 25 °. There is sex.

In the X-ray diffraction pattern shown in FIG. 1, the strongest peak is held at the position of 25.5 ° in all the samples having the mechanical milling processing time of 20 hours, 40 hours, and 50 hours, and the other 27.9 ° and 30.2 °. Diffraction peaks were confirmed at positions of, 38.0 °, 45.0 °, 50.1 °, 52.8 °, 53.8 °, 60.4 ° and 69.5 °. The plane spacing corresponding to the 25.5 ° diffraction peak is 3.49 Å, and the plane spacing corresponding to the 38.0 ° diffraction peak is 2.37 Å. The half widths of the peaks of 25.5 ° (full widths at half maximum) in Examples 1, 2 and 3 were 0.80 °, 0.81 ° and 0.84 °, respectively.

Next, when X-ray diffraction (XRD) by the transmission method is performed using high-energy X-rays, it is not easily affected by the sample surface (surface roughness, etc.) in a relatively low-angle diffraction experiment, and the atmosphere. The effect of unexposed folders can be reduced. _{The Li 4} SnS ₄ powder obtained in Example 1 was subjected to an X-ray diffraction (XRD) pattern using 38 keV X-rays. The results are shown in FIG. In order to avoid exposure of the measurement sample to the atmosphere, the sample was measured in a state of being packaged in a film manufactured by Kapton (registered trademark). In addition, information on short-range structures can be obtained by performing two-body correlation function analysis. _{For the Li 4} SnS ₄ powder obtained in Example 1, the two-body distribution function g (r) obtained by high-energy X-ray diffraction measurement is shown in FIG. The strongest peak (peak that can be attributed to Sn-S correlation) was found at 2.38 Å, and other peaks were also seen at 3.06 Å, 3.96 Å and 4.73 Å.

Next, the Li ₄ SnS ₄ powder obtained in Example 1 was observed with a scanning electron microscope (SEM). The results are shown in FIG. Particles of about 0.2 μm to 50 μm were confirmed, and it can be understood that the average particle size of _{Li 4} SnS _{4 powder is about 5 to 20 μm.}

[Test Example 3: Charging / Discharging Test]
_{Next, using the Li 4} SnS ₄ powder obtained in Example 1 as an electrode active material, an electrochemical cell was prepared by the following method, and constant current charge / discharge measurement was performed at a current density of 20 mA / g.

As a method for producing an electrochemical cell, first, the working electrode is acetylene black and polytetrafluoroethylene (PTFE), which is a binder, with respect _{to the Li 4} SnS _{4 powder obtained in Example 1, and Li 4} SnS ₄ The mixture was prepared by adding powder: acetylene black: PTFE = 86: 8: 6 in a mass ratio, kneading in a mortar for 15 minutes, and then attaching to an aluminum mesh. Polypropylene was used as the separator, and lithium was used as the counter electrode. The electrolytic solution is 1M lithium hexafluorophosphate (LiPF ₆ ) dissolved in a mixed solvent having a mass ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) of 1: 1 (1M LiPF ₆ EC / DMC). ) Was used.

Using the above-mentioned electrochemical cell, a charge / discharge test was performed by constant current charge / discharge measurement, but reversible charge / discharge was not possible. From this, it was suggested that the crystalline lithium tin sulfide of the present invention does not exhibit electrode activity in this potential range and has high electrochemical stability as a solid electrolyte.

[Example 4: Synthesis of _{Li 6} SnS _{5 powder]}
In a glove box in an argon atmosphere (non-exposed to the atmosphere), commercially available lithium sulfide (Li ₂ S) powder and tin sulfide (SnS ₂ ) powder are mixed in a molar ratio of 3: 1 (mass ratio 0.6448: 0.8552). Weighed and mixed, and then mechanical milling at 510 rpm for 80 hours using a 45 mL zirconia container containing about 500 (90 g) zirconia balls with a diameter of 4 mm using a ball mill device (Fritsch P-7, Classic Line). To _{obtain Li 6} SnS ₅ powder. The mechanical milling treatment was carried out in an atmosphere control overpot for a total of 80 times with a 1-hour milling treatment and a 15-minute pause. The obtained _{powder having a Li 4} SnS ₄ composition is referred to as Li ₆ SnS ₅ powder for convenience. The same applies to Examples 5 to 6. The powder density of the obtained Li ₆ SnS ₅ powder was measured with a dry powder density meter (Accupic II 1340-1CC manufactured by Makelomericix) and found to be 2.40 g cm ^-3 .

[Example 5: Synthesis of _{Li 6} SnS _{5 powder]}
In a glove box in an argon atmosphere (non-exposed to the atmosphere), commercially available lithium sulfide (Li ₂ S) powder and tin sulfide (SnS ₂ ) powder are mixed in a molar ratio of 3: 1 (mass ratio 0.6448: 0.8552). Weighed and mixed, and then mechanical milling at 510 rpm for 60 hours using a 45 mL zirconia container containing about 500 (90 g) zirconia balls with a diameter of 4 mm using a ball mill device (Fritsch P-7, Classic Line). To _{obtain Li 6} SnS ₅ powder. In the mechanical milling process, the milling process for 1 hour was performed 60 times in total in the overpot for atmosphere control with a pause of 15 minutes.

[Example 6: Synthesis of _{Li 6} SnS _{5 powder]}
In a glove box in an argon atmosphere (non-exposed to the atmosphere), commercially available lithium sulfide (Li ₂ S) powder and tin sulfide (SnS ₂ ) powder are mixed in a molar ratio of 3: 1 (mass ratio 0.6448: 0.8552). Weighed and mixed, and then mechanical milling at 510 rpm for 20 hours using a 45 mL zirconia container containing about 500 (90 g) zirconia balls with a diameter of 4 mm using a ball mill device (Fritsch P-7, Classic Line). To _{obtain Li 6} SnS ₅ powder. The mechanical milling treatment was carried out in an atmosphere control overpot for a total of 20 times with a 1-hour milling treatment and a 15-minute pause.

[Test Example 4: Measurement of Conductivity of Solid Electrolyte Pellet (Part 2)]
_{80 mg of the Li 6} SnS ₅ powder obtained in Example 4 was homogeneously filled in a molding machine having a diameter of 10 mm, and uniaxially molded at 330 MPa for 1 minute to prepare pellets having a thickness of 0.564 mm and a diameter of 10.00 mmΦ. When the density of this pellet was calculated from the shape, it was 1.81 gcm ^-3 (relative density 75.3%). The resistance of this pellet at 25 ° C was measured by the AC impedance method and found to be 3,750 Ω, and the ionic conductivity σ of this pellet at 25 ° C was 1.92 × 10 ^-5 Scm ^-1 . The electron resistance value measured by the DC polarization method is at least 1 MΩ or more, the electron conductivity is at least 7 × 10 ^-8 Scm ^-1 or less, and the electron conductivity is 1/250 or less of the ionic conductivity. Do you get it.

[Test Example 5: X-ray structure diffraction (2)]
The X-ray diffraction (XRD) pattern using CuKα rays was measured for the powders obtained in Examples 4 to 6. The results are shown in FIG. For reference, FIG. 5 also shows the peaks of the sample of Example 1, lithium sulfide (Li ₂ S) and tin sulfide (SnS _{2) used as raw materials.} In addition, in the XRD measurement, in order to avoid exposure to the atmosphere of the prepared sample, a polymer-made air-unexposed folder was used, so it is possible that some effects of the polymer were confirmed around 2θ = 10 to 25 °. There is sex.

In the X-ray diffraction pattern shown in FIG. 5, the mechanical milling treatment time of the samples of 20 hours, 60 hours, and 80 hours all had the strongest peak at the position of 27.0 ° and the second at the position of 25.5 °. It has a strong peak at 31.3 °, 38.1 °, 44.9 °, 50.1 °, 53.1 °, 55.6 °, 65.3 ° and 72.0 °. In addition, the existence of a diffraction peak was suggested around 27.9 °. Furthermore, the plane spacing corresponding to the 25.5 ° diffraction peak is 3.49 Å, and the plane spacing corresponding to the 38.1 ° diffraction peak is 2.36 Å. The half widths (full widths at half maximums) of the 25.5 ° peaks in Examples 4, 5 and 6 were 0.92 °, 1.03 ° and 1.05 °, respectively.

As described above, in Examples 4 to 6, a two-phase structure of a crystal phase _{composed of Li 4} SnS ₄ and a crystal phase composed of Li _{2 S was formed.} From the charged composition ratio, _{it is expected that the abundance ratio of the Li 4} SnS ₄ phase and the Li ₂ S phase is about 50:50 (mol ratio). The presence ratio of the Li ₄ SnS ₄ and Li ₂ S calculated from the density of the Li ₄ SnS ₄ and Li ₂ S 50: density of the two-phase mixture of 50 (mol ratio), with 2.41 g cm ^-3 Yes, the dry powder density of Li _{6 SnS 5} ^{powder is 2.40 g cm -3, which} is a good match. At this abundance ratio, _{the volume ratio of Li 4} SnS ₄ and Li ₂ S is approximately 79:21, and the mass ratio is 86:14. As described above, _{since the crystal phase composed of Li 4} SnS ₄ is a crystal phase having excellent ionic conductivity, the samples obtained in Examples 4 to 6 also have excellent ionic conductivity and are useful as a solid electrolyte.

Claims (9)

Contains lithium, tin and sulfur as constituent elements
General formula (1):
Li _n1 SnS _m1 (1)
[In the equation, n1 and m1 satisfy 3.5 ≤ n1 ≤ 4.5; 3.5 ≤ m1 ≤ 4.5. ]
Represented by
Within the range of diffraction angle 2θ = 10 ° to 80 ° in the X-ray diffraction diagram by CuKα rays, it has the strongest peak or the second strongest peak at least at 25.5 ° and 24.5 within the allowable range of ± 1.0 °. Crystalline lithium tin sulfide with no peak below °. A crystalline lithium-tin sulfide according to claim 1, the composition ratio of lithium and sulfur (Li / S) has a two-phase structure of the lithium sulfide is 1.0 to 3.0 molar ratio, sintering-crystalline lithium Lithium sulfide. The crystalline lithium tin sulfide according to claim 1 or 2 , wherein the half width of the peak at 2θ = 25.5 ° is 0.20 to 1.05 °. Claim that the diffraction angle in the X-ray diffraction pattern by CuKα rays is within the range of 2θ = 10 ° to 80 °, the tolerance is ± 1.0 °, and the peaks are 27.9 °, 38.0 °, 45.0 ° and 50.1 °. The crystalline lithium tin sulfide according to any one of 1 to 3. The crystalline lithium tin sulfide according to any one of claims 1 to 4 , which has an average particle size of 1 to 30 μm. The method for producing a crystalline lithium tin sulfide according to any one of claims 1 to 5 , wherein the method comprises a step of using lithium sulfide and tin sulfide as raw materials and subjecting them to a mechanical milling treatment. The production method according to claim 6 , wherein the amount of lithium sulfide used is 1.75 to 3.5 mol with respect to 1 mol of tin sulfide. A solid electrolyte for a lithium ion secondary battery made of the crystalline lithium tin sulfide according to any one of claims 1 to 5. A lithium ion secondary battery using the crystalline lithium tin sulfide according to any one of claims 1 to 5 or the solid electrolyte for a lithium ion secondary battery according to claim 8.

Images