Low-crystalline vanadium sulfide

Abstract

This invention provides an electrode active material for lithium-ion secondary batteries that exhibits excellent capacity. [Solution] A low-crystallinity vanadium sulfide, It contains vanadium and sulfur as constituent elements, The composition ratio (S / V) of vanadium to sulfur is 3.5 or more in molar ratio. The average particle size D50 is 10 μm or less. In the X-ray diffraction pattern using CuKα rays, there is no peak at 2θ = 53.0° within a tolerance of ±1.0°. Low-crystalline vanadium sulfide.

Description

[Technical Field]

[0001] This invention relates to low-crystalline vanadium sulfide. [Background technology]

[0002] In recent years, the increasing performance of portable electronic devices and hybrid vehicles has led to a growing demand for higher capacity lithium-ion secondary batteries used in them. However, current lithium-ion secondary batteries suffer from insufficient capacity enhancement of the positive electrode compared to the negative electrode, and even lithium nickel oxide-based materials, which are considered to have relatively high capacity, only achieve a capacity of around 190-220 mAh / g.

[0003] On the other hand, sulfur has a high theoretical capacity of approximately 1670 mAh / g and is expected to be used as a cathode material. However, it has low electronic conductivity and also dissolves into the organic electrolyte as lithium polysulfide during charging and discharging, making technology to suppress dissolution into the organic electrolyte essential.

[0004] Although metal sulfides possess electronic conductivity and do not dissolve easily into organic electrolytes, they have a lower theoretical capacity compared to sulfur, and furthermore, they suffer from low reversibility due to large structural changes associated with Li insertion and removal during charging and discharging. Increasing the sulfur content is necessary to achieve higher capacity in metal sulfides, but in crystalline metal sulfides, the maximum capacity is determined by the crystal space group that determines the sites where Li is inserted during discharge, making it difficult to exceed this maximum capacity value.

[0005] For example, among metal sulfides, vanadium sulfide is sold as a reagent, such as crystalline vanadium(III)(V) sulfide. 2.0 S 3.0When ) is used as the positive electrode active material, although the theoretical capacity is high at 811.0 mAh / g, the measured capacity is only about 52 mAh / g for discharge capacity and about 23 mAh / g for charge capacity because the reaction with the organic electrolyte cannot be suppressed. Non-patent document 1 also proposes VS4-rGO, a nanocomposite with graphene oxide produced by hydrothermal synthesis, as a negative electrode material for lithium-ion secondary batteries due to its high output characteristics. However, since this material is crystalline, even when a nanocomposite is formed with other materials, the maximum capacity is determined by the crystal space group that determines the sites where Li is inserted during discharge, so there is room for improvement in capacity.

[0006] From this perspective, the present inventors conducted diligent research and found that a certain low-crystalline vanadium sulfide can improve the actual capacity (see Patent Document 1). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2018 / 181698 [Non-patent literature]

[0008] [Non-Patent Document 1] X. Xu, et al., J. Mater. Chem. A, 2. (2014) 10847-10853. [Overview of the project] [Problems that the invention aims to solve]

[0009] However, in the low-crystalline vanadium sulfide described above, it was not possible to sufficiently remove impurities (e.g., oxides such as V2O3). Therefore, although vanadium sulfide has the potential to produce high-capacity lithium-ion secondary batteries due to its large theoretical capacity, there was still room to improve the measured capacity.

[0010] Based on the above, the present invention aims to provide an electrode active material for lithium-ion secondary batteries that exhibits excellent capacity. [Means for solving the problem]

[0011] The inventors have diligently conducted research to achieve the above-mentioned objectives. As a result, they have found that by applying specific reaction conditions, a uniform and fine low-crystalline vanadium sulfide can be obtained, and that this low-crystalline vanadium sulfide exhibits high capacity when used as an electrode active material for lithium-ion secondary batteries. The present invention was completed based on these findings and further research. That is, the present invention encompasses the following configuration.

[0012] Item 1. Low-crystallinity vanadium sulfide, It contains vanadium and sulfur as constituent elements, The composition ratio (S / V) of vanadium to sulfur is 3.5 or more in molar ratio. The average particle size D50 is 10 μm or less. In the X-ray diffraction pattern using CuKα rays, there is no peak at the diffraction angle 2θ = 53.0° within a tolerance of ±1.0°. Low-crystalline vanadium sulfide.

[0013] Section 2. General formula (1): VS x (1) [In the equation, x is 3.5 or greater.] A low-crystalline vanadium sulfide according to item 1, having the composition represented by .

[0014] Item 3. A low-crystallinity vanadium sulfide according to item 1 or 2, having a VS4 type crystal structure.

[0015] Item 4. In the X-ray diffraction pattern, peaks are present at 2θ = 15.8° and 17.0° within a tolerance of ±1.0°. The crystallite sizes respectively obtained from the half-value widths of the peaks at 2θ = 15.8° and 17.0° are each 15 nm or less, the low-crystalline vanadium sulfide according to any one of items 1 to 3.

[0016] Item 5. In the X-ray diffraction pattern, within an allowable range of ±1.0°, does not have peaks at 2θ = 24.4° and 33.0°, or the areas of the peaks having maxima at 2θ = 24.4° and 33.0° are each 20% or less of the smaller one of the areas of the two peaks having maxima at 2θ = 15.8° and 17.0°, the low-crystalline vanadium sulfide according to any one of items 1 to 4.

[0017] Item 6. A method for producing a low-crystalline vanadium sulfide according to any one of items 1 to 5, a hydrothermal treatment step of hydrothermally treating a raw material containing a vanadate and a sulfur-containing compound at 100°C to 160°C in the absence of a carrier comprising the production method.

[0018] Item 7. The production method according to item 6, wherein the usage amount of the sulfur-containing compound is 3.5 mol to 5.0 mol with respect to 1 mol of the vanadate.

[0019] Item 8. An electrode active material for a lithium-ion secondary battery comprising the low-crystalline vanadium sulfide according to any one of items 1 to 5.

[0020] Item 9. An electrode for a lithium-ion secondary battery comprising the electrode active material for a lithium-ion secondary battery according to item 8.

[0021] Item 10. A lithium-ion secondary battery comprising the electrode for a lithium-ion secondary battery according to item 9.

Advantages of the Invention

[0022] The low-crystalline vanadium sulfide of the present invention is a material capable of significantly improving the charge and discharge capacity as compared with conventional vanadium sulfides. [Brief explanation of the drawing]

[0023] [Figure 1] The X-ray diffraction patterns of the powders obtained in Example 1, Comparative Example 1, and Comparative Example 2 are shown, along with the peaks for crystalline vanadium(IV) tetrasulfide (VS4) and crystalline divanadium trioxide (V2O3). [Figure 2] The HR-TEM image of the powder obtained in Example 1 is shown. [Figure 3] The HR-TEM image of the powder obtained in Comparative Example 1 is shown. [Figure 4] The HR-TEM image of the powder obtained in Comparative Example 2 is shown. [Figure 5] The particle size distribution (histogram) of the powder obtained in Example 1 is shown. [Figure 6] The discharge ratio capacities of the powders obtained in Example 1 and Comparative Example 2 at 25°C and 60°C are shown. [Modes for carrying out the invention]

[0024] In this specification, "contains" is a concept that encompasses all of the following: "contains," "consist essentially of," and "consist of."

[0025] Furthermore, in this specification, the notation "A~B" means "greater than or equal to A and less than or equal to B".

[0026] Furthermore, in this specification, the term "lithium-ion secondary battery" is a concept that also includes lithium secondary batteries in which lithium metal is used as the negative electrode active material.

[0027] 1. Low-crystalline vanadium sulfide The low-crystallinity vanadium sulfide of the present invention is It contains vanadium and sulfur as constituent elements, The composition ratio (S / V) of vanadium to sulfur is 3.5 or more in molar ratio. The average particle size D50 is 10 μm or less. In the X-ray diffraction pattern using CuKα rays, there is no peak at the diffraction angle 2θ = 53.0° within a tolerance of ±1.0°. It is a low-crystalline vanadium sulfide.

[0028] More specifically, the low-crystalline vanadium sulfide of the present invention has a general formula (1): VS x (1) [In the equation, x is 3.5 or greater.] It is preferable to have a composition represented by .

[0029] Thus, the low-crystalline vanadium sulfide of the present invention has a high elemental ratio of sulfur to vanadium. For this reason, the low-crystalline vanadium sulfide of the present invention has a high charge-discharge capacity. In this invention, the higher the sulfur content (the larger x is), the higher the charge-discharge capacity tends to be, and the lower the sulfur content (the smaller x is), the less elemental sulfur is contained, and the better the cycle characteristics tend to be. From the viewpoint of balancing these, x is preferably 3.5 to 10.0, more preferably 3.8 to 5.0, and even more preferably 3.9 to 4.5.

[0030] The low-crystallinity vanadium sulfide of the present invention preferably has an average particle diameter D50 of 8 μm or less, and more preferably 6 μm or less. The smaller the average particle diameter D50 of the low-crystallinity vanadium sulfide of the present invention, the better; there is no particular lower limit, but for example, it can be 50 nm or more. Here, the average particle diameter D50 only needs to be within the above range for the average particle diameter of primary particles, and the average particle diameter of secondary particles may also be within the above range.

[0031] In this invention, the average particle diameter D50 is determined by laser diffraction and scattering, for example, under the following measurement conditions: Device: AEROTRAC II (Microtrac bell) Conditions: Discharge pressure 0.3 MPa It can be measured using [this method].

[0032] The low-crystalline vanadium sulfide of the present invention preferably has a crystal structure similar to that of crystalline vanadium(IV) tetrasulfide (VS4) (hereinafter sometimes referred to as "VS4-type crystal structure"). Of course, it is particularly preferable to use VS4 as the vanadium sulfide. This crystalline vanadium(IV) tetrasulfide (VS4) is listed as 16797 in the Inorganic Crystal Structure Database (ICSD).

[0033] The low-crystallinity vanadium sulfide of the present invention preferably has peaks at 2θ = 15.8° and 17.0° within a tolerance of ±1.0° in the diffraction angle range 2θ = 10° to 80° in the X-ray diffraction pattern using CuKα rays. Having peaks at 2θ = 15.8° and 17.0° within a tolerance of ±1.0° means having two peaks, one with a maximum in the range 2θ = 14.8° to 16.8° and the other with a maximum in the range 2θ = 16.0° to 18.0°. If the positions of these two peaks become unclear due to overlap, peak splitting using a Gaussian function is performed to extract the positions of each peak.

[0034] The low-crystallinity vanadium sulfide of the present invention preferably has peaks at at least one location (especially all) at 2θ = 30.1° and 36.5° within a tolerance of ±1.0° in the diffraction angle range 2θ = 10° to 80° in the X-ray diffraction pattern using CuKα rays. Having peaks at at least one location at 2θ = 30.1° and 36.5° within a tolerance of ±1.0° means having peaks with maxima in at least one range (especially all) of the range 2θ = 29.1° to 31.1° and the range 2θ = 35.5° to 37.5°.

[0035] The low-crystallinity vanadium sulfide of the present invention does not have a peak at 2θ=53.0° within a tolerance of ±1.0° in the diffraction angle range 2θ=10° to 80° in the CuKα X-ray diffraction pattern. The absence of a peak at 2θ=53.0° within a tolerance of ±1.0° in the CuKα X-ray diffraction pattern means that there is no peak in the range of 2θ=52.0° to 54.0°. This peak is characteristic of crystalline divanadium trioxide (V2O3). Thus, the low-crystallinity vanadium sulfide of the present invention has a reduced content of impurities (e.g., oxides such as V2O3) that could not be sufficiently removed conventionally. Therefore, the low-crystallinity vanadium sulfide of the present invention has a high charge-discharge capacity.

[0036] In addition, it is preferable that the low-crystallinity vanadium sulfide of the present invention either does not have peaks at 2θ = 24.4° and 33.0°, which are typical peak positions found in crystalline vanadium trioxide (V2O3), within a tolerance of ±1.0°, or that the area of ​​each peak with a maximum at these positions is 20% or less of the smaller of the areas of the two peaks with maximums at 2θ = 15.8° and 17.0°. In other words, it is preferable that the low-crystalline vanadium sulfide of the present invention does not have a peak with a maximum in at least one of the ranges 2θ=23.4°~25.4° and 2θ=32.0°~34.0° (especially all of them), or that the area of ​​each of the peaks with a maximum in the ranges 2θ=23.4°~25.4° and 2θ=32.0°~34.0° is 20% or less (0%~20%, especially 0.1%~19%) of the smaller of the areas of the two peaks with a maximum in the ranges 2θ=14.8°~16.8° and 2θ=16.0°~18.0°. Thus, it is preferable that the low-crystalline vanadium sulfide of the present invention does not have a crystal structure similar to crystalline vanadium trioxide (V2O3).

[0037] The low-crystallinity vanadium sulfide of the present invention preferably has a crystallite size of 15 nm or less, and more preferably 10 nm or less. While the crystallite size of the low-crystallinity vanadium sulfide of the present invention is desirable as small as possible, there is no specific lower limit; however, it can be, for example, 1 nm or more (e.g., 5 nm or more).

[0038] In this invention, the crystallite size is determined within a tolerance of ±1.0° from the peaks at 2θ = 15.8° ((11-1) plane) and 17.0° ((020) plane) in the above X-ray diffraction pattern, and is calculated from the full width at half maximum of these peaks using Scherrer's formula. More specifically, the crystallite size is given by the formula: D = Kλ / Bcosθ [In the formula, D is the crystallite size, λ is the wavelength of the X-ray, B is the full width at half maximum (FMAX) of the peak at 2θ = 15.4° or 17.0°, θ is the diffraction angle, and K is the Scherrer constant. In this invention, K is set to 1.0. However, if the FMAX B of the two peaks at 2θ = 15.4° or 17.0° overlap and the FMAX of those two peaks becomes unclear, peak splitting using a Gaussian function is performed to extract the FMAX of each peak.] It is determined by [method].

[0039] In this invention, the X-ray diffraction pattern is obtained by powder X-ray diffraction measurement, for example, under the following measurement conditions: Equipment: RINT TTR-III (Rigaku) X-ray source: CuKα 50kV / 300mA Measurement conditions: 2θ = 10° to 80°, 0.1° step, scanning speed 0.02° / sec It can be measured using [this method].

[0040] Furthermore, it is preferable that the low-crystallinity vanadium sulfide of the present invention does not show crystal lattice fringes originating from crystal domains in its high-resolution transmission electron microscope (HR-TEM) observation image.

[0041] In this invention, high-resolution transmission electron microscopy (HR-TEM) observation is performed under the following measurement conditions, for example: Equipment: Talos F200X (ThermoFisher Scientific) Conditions: 300kV and 300pA It can be measured using [this method].

[0042] The low-crystallinity vanadium sulfide of the present invention, despite having a high proportion of sulfur in its average composition, contains almost no elemental sulfur, as described below. Instead, it combines with vanadium to form a low-crystallinity sulfide. Thus, by reducing its crystallinity, the low-crystallinity vanadium sulfide of the present invention has many sites where lithium ions can be inserted and removed, and structurally possesses gaps that can serve as three-dimensional conductive pathways for lithium. It also has numerous advantages, such as the ability to undergo three-dimensional volume changes during charging and discharging. Therefore, the charge and discharge capacity can be further improved. In this specification, the average composition of the sulfide refers to the elemental ratio of each element constituting the entire sulfide.

[0043] The following describes the "low crystallinity" in the present invention. In the low-crystallinity vanadium sulfide of the present invention, it is preferable that the full width at half maximum (FWHM) of the peaks at 2θ = 15.8° and 17.0° are both between 0.3° and 2.5° (particularly between 0.4° and 2.3°). In other words, it is preferable that the low-crystallinity vanadium sulfide of the present invention has a larger FWHM of the peaks at 2θ = 15.8° and 17.0° compared to crystalline vanadium(IV) tetrasulfide (VS4). Thus, because the low crystallinity of the low-crystallinity vanadium sulfide of the present invention increases the number of sites where Li can exist stably, the charge / discharge capacity and cycle characteristics can be improved. If the FWHM of the two peaks at 2θ = 15.4° or 17.0° overlap and the FWHM of these two peaks becomes unclear, peak splitting using a Gaussian function is performed to extract the FWHM of each peak.

[0044] Furthermore, when materials containing large amounts of elemental sulfur are used as electrode active materials (such as positive electrode active materials), elemental sulfur has high resistance, which increases the resistance of the electrode. This raises concerns that the utilization rate of the electrode active material will decrease during the charge-discharge reaction, resulting in insufficient capacity. In contrast, the low-crystalline vanadium sulfide of the present invention contains almost no elemental sulfur, so when used as an electrode active material, the problem of reduced utilization rate does not occur, and a higher energy density can be obtained.

[0045] More specifically, the strongest peak for sulfur (S8) is located at 2θ = 23.0° within a tolerance of ±1.0°. Therefore, it is preferable that, in the X-ray diffraction pattern using CuKα rays, within a tolerance of ±1.0°, there is no peak with a maximum at 2θ = 23.0°, which is characteristic of elemental sulfur, or that the area of ​​the peak with a maximum at 2θ = 23.0° is 20% or less (0% to 20%, particularly 0.1% to 19%) of the area of ​​the smaller of the two peaks with maximums at 2θ = 15.8° and 17.0°. This makes it possible to create a low-crystallinity vanadium sulfide of the present invention that contains almost no elemental sulfur, thereby reducing concerns about the reduced utilization rate of the electrode active material as described above and further improving the charge-discharge capacity.

[0046] In addition, it is preferable that the low-crystallinity vanadium sulfide of the present invention does not have peaks at the positions 2θ=25.8° and 27.8°, which are characteristic peaks of elemental sulfur, within a tolerance range of ±1.0°, or that the area of ​​the peak with a maximum at these positions is 10% or less (0-10%, particularly 0.1-8%) of the area of ​​the smaller of the peaks with a maximum at 2θ=15.8° and 17.0°. This makes it possible to make the low-crystallinity vanadium sulfide of the present invention a material that contains almost no elemental sulfur, thereby reducing concerns about the decrease in the utilization rate of the electrode active material as described above and further improving the charge-discharge capacity.

[0047] In this invention, other impurities may be included as long as they do not impair the performance of the low-crystallinity vanadium sulfide of the present invention, but it is preferable that no impurities other than the above-mentioned components are substantially included. Examples of such impurities include vanadium oxides (e.g., V2O3, etc.) that are known to be normally produced as by-products in the vanadium sulfide raw material and in the prior art, vanadium that may be mixed into the raw material, and oxygen that may be mixed into the raw material and during manufacturing. The amount of these impurities is preferably within a range that does not impair the performance of the low-crystallinity vanadium sulfide of the present invention as described above, and is usually preferably 2% by mass or less (0% to 2% by mass), and more preferably 1.5% by mass or less (0% to 1.5% by mass). However, as described above, it is preferable that elemental sulfur is included as little as possible as an impurity. In this way, the content of impurities (e.g., oxides such as V2O3, etc.) that could not be sufficiently removed in the conventional low-crystallinity vanadium sulfide of the present invention has been reduced. For this reason, the low-crystallinity vanadium sulfide of the present invention has a high charge-discharge capacity. Furthermore, the low-crystallinity vanadium sulfide of the present invention has the advantage of not requiring any process to remove impurities (for example, heat treatment under a reducing atmosphere) and the necessary equipment, since the impurity content has already been sufficiently reduced.

[0048] The low-crystallinity vanadium sulfide of the present invention that satisfies these conditions has a strong peak at g(r) = 2.4 Å within an acceptable range of ±0.1 Å in X-ray neutron atom-pair correlation function analysis (PDF analysis). However, for sulfides with better charge / discharge capacity and cycle characteristics, it is preferable to have a shoulder peak at g(r) = 2.0 Å, and also a peak at g(r) = 3.3 Å. In other words, it is preferable that the low-crystallinity vanadium sulfide of the present invention has not only VS bonds but also SS bonds (disulfide bonds).

[0049] As described above, the low-crystallinity vanadium sulfide of the present invention has particularly excellent charge and discharge capacity, making it useful as an electrode active material for lithium-ion secondary batteries (especially a positive electrode active material for lithium-ion secondary batteries).

[0050] 2. Method for producing low-crystallinity vanadium sulfide The low-crystallinity vanadium sulfide of the present invention is A hydrothermal treatment process in which raw materials containing vanadate and sulfur-containing compounds are hydrothermally treated at 100°C to 160°C in the absence of a support. It can be obtained by a manufacturing method that includes [the specified features].

[0051] Specific raw materials include vanadates such as ammonium vanadate (NH4VO3), potassium vanadate (KVO3), and sodium vanadate (NaVO3). Among these, ammonium vanadate (NH4VO3) is preferred. There are no particular limitations on the vanadate, and any commercially available vanadate can be used. It is especially preferable to use a high-purity vanadate.

[0052] Furthermore, as sulfur-containing compounds, a wide range of compounds can be used that contain the amount of sulfur necessary to form a sulfide of the desired composition, and that produce the sulfide when hydrolyzed after reaction with a substrate. Examples of sulfur-containing compounds include thioacetamide (C2H5NS) and ammonium sulfide ((NH4)2S). Among these, thioacetamide (C2H5NS) is preferred. There are no particular limitations on the sulfur-containing compound used as a raw material, and any commercially available sulfur-containing compound can be used. In particular, it is preferable to use one of high purity.

[0053] Regarding the mixing ratio of the raw materials, since the charging ratio of the raw materials is almost directly proportional to the ratio of each element in the product, the ratio can be the same as the elemental ratio of vanadium and sulfur in the target low-crystalline vanadium sulfide. The amount of sulfur-containing compound used is not particularly limited, but from the viewpoint of charge / discharge capacity and cycle characteristics, it is preferable to use 3.5 moles or more of the sulfur-containing compound per mole of vanadate, more preferably 3.5 to 10.0 moles, even more preferably 3.8 to 5.0 moles, and particularly preferably 3.9 to 4.5 moles.

[0054] From the viewpoint of charge / discharge capacity and cycle characteristics, a temperature of 100°C to 140°C is preferred for the hydrothermal treatment process.

[0055] Furthermore, the pressure in the hydrothermal treatment process is preferably 0.8 atmospheres or higher, and more preferably 1 to 2 atmospheres, from the viewpoint of charge / discharge capacity and cycle characteristics.

[0056] There are no particular limitations on the duration of the hydrothermal treatment process; the heating treatment can be performed for any amount of time until the desired low-crystallinity vanadium sulfide precipitates. For example, the hydrothermal treatment can be performed within a range of 0.1 hours to 100 hours (especially 1 hour to 24 hours).

[0057] Thus, it is preferable that the hydrothermal treatment step be carried out in a raw material liquid containing the above-mentioned raw materials. In this specification, hydrothermal reaction means a chemical reaction that occurs in a chemical synthesis treatment (hydrothermal treatment) carried out in the presence of hot water or steam at a temperature of 100°C or higher and a pressure of 0.8 atmospheres or higher (particularly 1 atmosphere or higher).

[0058] By performing the hydrothermal treatment process in the absence of a support, low-crystallinity vanadium sulfide with small particle size and low crystallinity can be obtained. Examples of support materials include carbon support materials such as carbon black, graphite powder, carbon nanotubes, and graphene plates; and ceramic support materials such as alumina support materials, silica support materials, zirconia support materials, and titania support materials.

[0059] 3. Uses of low-crystallinity vanadium sulfide As described above, the low-crystallinity vanadium sulfide of the present invention is particularly useful as an electrode active material for lithium-ion secondary batteries because it exhibits excellent charge and discharge capacity. A lithium-ion secondary battery that can effectively use the low-crystallinity vanadium sulfide of the present invention as an electrode active material (especially a positive electrode active material) may be a non-aqueous electrolyte lithium-ion secondary battery using a non-aqueous electrolyte, or an all-solid-state lithium-ion secondary battery using a lithium-ion conductive solid electrolyte.

[0060] The structure of non-aqueous electrolyte lithium-ion secondary batteries and all-solid-state lithium-ion secondary batteries can be the same as that of known lithium-ion secondary batteries, except that the low-crystallinity vanadium sulfide of the present invention is used as the electrode active material (especially the positive electrode active material).

[0061] For example, a non-aqueous electrolyte lithium-ion secondary battery can have the same basic structure as a known non-aqueous electrolyte lithium-ion secondary battery, except that the low-crystallinity vanadium sulfide of the present invention is used as the electrode active material (especially the positive electrode active material).

[0062] Regarding the positive electrode, when the low-crystallinity vanadium sulfide of the present invention is used as the positive electrode active material, the structure can be the same as that of known positive electrodes, except that the low-crystallinity vanadium sulfide of the present invention is used as the positive electrode active material. For example, a positive electrode mixture containing the low-crystallinity vanadium sulfide of the present invention 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. As the conductive agent, for example, carbon materials such as graphite, coke, carbon black (Ketjenblack, etc.), and needle-shaped carbon can be used. As the binder, for example, materials such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyimide (PI), polyamide, polyamide-imide, polyacrylic, styrene-butadiene rubber (SBR), styrene-ethylene-butylene-styrene copolymer (SEBS), and carboxymethylcellulose (CMC) can be used individually or in combination of two or more. On the other hand, if the low-crystalline vanadium sulfide of the present invention is not used as the positive electrode active material, known positive electrode active materials such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), vanadium oxide-based materials, and sulfur-based materials can also be used as the positive electrode active material.

[0063] Regarding the negative electrode, when the low-crystallinity vanadium sulfide of the present invention is used as the negative electrode active material, the structure can be the same as that of known negative electrodes, except that the low-crystallinity vanadium sulfide of the present invention is used as the negative electrode active material. For example, a negative electrode mixture containing the low-crystallinity vanadium sulfide of the present invention and, if necessary, 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, carbon materials such as graphite, coke, carbon black, or needle-shaped carbon can be used. As the binder, for example, materials such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyimide (PI), polyamide, polyamide-imide, polyacrylic, styrene-butadiene rubber (SBR), styrene-ethylene-butylene-styrene copolymer (SEBS), and carboxymethylcellulose (CMC) can be used individually or in combination of two or more. On the other hand, if the low-crystalline vanadium sulfide of the present invention is not used as the negative electrode active material, known negative electrode active materials such as metallic lithium, carbon-based materials (activated carbon, graphite, etc.), silicon, silicon oxide, Si-SiO-based materials, and lithium titanium oxide can also be used as the negative electrode active material.

[0064] As the solvent for the non-aqueous electrolyte, solvents known as solvents for non-aqueous lithium-ion secondary batteries, such as carbonates, ethers, nitriles, and sulfur-containing compounds, can be used. In particular, when elemental sulfur is used as the positive electrode active material, carbonates cannot be used as a solvent because they react with elemental sulfur, and ethers cannot be used as a solvent because a large amount of sulfur components dissolve in the electrolyte, causing performance deterioration. However, by using the low-crystallinity vanadium sulfide of the present invention as the electrode active material (especially the positive electrode active material), these problems can be solved, making it possible to use any solvent and improving the selectivity of the solvent in the electrolyte.

[0065] As a separator, for example, materials such as polyethylene, polypropylene or other polyolefin resins, fluororesins, nylon, aromatic aramid, and inorganic glass can be used, and materials in the form of porous membranes, nonwoven fabrics, woven fabrics, etc.

[0066] On the other hand, all-solid-state lithium-ion secondary batteries can have the same structure as known all-solid-state lithium-ion secondary batteries, except that the low-crystallinity vanadium sulfide of the present invention is used as the electrode active material (especially the positive electrode active material). In this case, the above-mentioned materials can be used as the positive electrode, negative electrode, and separator.

[0067] In this case, as the electrolyte, polymer-based solid electrolytes such as polyethylene oxide-based polymer compounds; polymer compounds containing at least one of polyorganosiloxane chains and polyoxyalkylene chains; as well as sulfide-based solid electrolytes, oxide-based solid electrolytes, etc., can also be used.

[0068] There are no particular limitations on the shape of non-aqueous electrolyte lithium-ion secondary batteries and all-solid-state lithium-ion secondary batteries; cylindrical, prismatic, or other shapes can be used. [Examples]

[0069] The present invention will be described in more detail below with reference to examples. However, it goes without saying that the present invention is not limited to the following examples.

[0070] [Example 1: Hydrothermal Treatment] Ammonium vanadate (NH4VO3) and thioacetamide (C2H5NS) were mixed in a molar ratio (NH4VO3:C2H5NS) of 1:3.5 to 5.0. The resulting mixture was hydrothermally treated at 100°C to 160°C for 16 hours in the absence of a support, and then washed with water and isopropanol to obtain vanadium sulfide powder.

[0071] [Comparative Example 1: Firing Treatment] Crystalline vanadium(III) sulfide (V2S3) and sulfur powder (S) were mixed in a molar ratio (V2S3:S) of 1:6, and the mixture was vacuum-sealed and calcined at 400°C for 5 hours. Subsequently, crystalline vanadium sulfide VS4 powder (c-VS4) was obtained by desulfurization at 200°C for 8 hours.

[0072] [Comparative Example 2: Milling Process] In a glove box under an argon atmosphere (not exposed to air), a 45 mL zirconia container containing 2.0 g of crystalline vanadium sulfide powder (c-VS4) obtained in Comparative Example 1 and approximately 500 zirconia balls (90 g) with a diameter of 4 mm was subjected to mechanical milling at 270 rpm for 40 hours using a ball mill (Fritsch P-7, Classic Line) to obtain amorphous vanadium sulfide VS4 powder (a-VS4). The above mechanical milling process was carried out in an atmosphere-controlling overpot, with 1 hour milling cycles followed by 15-minute breaks, for a total of 40 cycles.

[0073] [Test Example 1: X-ray Diffraction] Regarding the powders obtained in Example 1, Comparative Example 1, and Comparative Example 2, Measuring device: RINT TTR-III (Rigaku) X-ray source: CuKα 50kV / 300mA Measurement conditions: 2θ = 10° to 80°, 0.1° step, scanning speed 0.02° / sec X-ray diffraction (XRD) was measured using [a specific method]. The results are shown in Figure 1. For reference, Figure 1 also shows the peaks for crystalline vanadium(IV) sulfide (VS4) and the impurity crystalline vanadium trioxide (V2O3) powder.

[0074] In the X-ray diffraction pattern shown in Figure 1, peak splitting revealed that the powder of Example 1 had peaks at 2θ = 15.8° and 17.0° within the diffraction angle range of 2θ = 10° to 80°, showing a pattern similar to VS4. Furthermore, the full width at half maximum of these peaks was 1.12° and 1.12°, respectively, and the crystallite sizes determined based on these values ​​were D. (11-1) =8.0nm, D(020) It was 8.0 nm. On the other hand, in the X-ray diffraction pattern shown in Fig. 1, no peak was present around 2θ = 53.0°.

[0075] Also, in the X-ray diffraction pattern shown in Fig. 1, the powder of Comparative Example 1 had peaks at 2θ = 15.8° and 17.0° within the range of diffraction angle 2θ = 10° to 80°. Also, the full width at half maximum of these peaks was 0.22° and 0.22° respectively, and the crystallite sizes obtained based on these were D (11-1) = 41 nm, D (020) = 41 nm. On the other hand, in the X-ray diffraction pattern shown in Fig. 1, no peak was present around 2θ = 53.0°.

[0076] Also, when peak splitting was performed in the X-ray diffraction pattern shown in Fig. 1, the powder of Comparative Example 2 had peaks at 2θ = 15.8° and 17.0° within the range of diffraction angle 2θ = 10° to 80°. Also, the full width at half maximum of these peaks was 2.9° and 2.9° respectively, and the crystallite sizes obtained based on these were D (11-1) = 3.1 nm, D (020) = 3.1 nm. Furthermore, in the X-ray diffraction pattern of Comparative Example 2, peaks were present at 2θ = 24.4°, 33.0°, and 54.0°. Therefore, it was confirmed that the powder of Comparative Example 2 contained a crystal structure similar to that of crystalline vanadium trioxide (V2O3) which is an impurity.

[0077] [Test Example 2: High-Resolution Transmission Electron Microscope (HR-TEM) Observation] For the powders obtained in Example 1, Comparative Example 1, and Comparative Example 2, Measuring device: Talos F200X (ThermoFisher Scientific) Accelerating voltage: 300 kV and 300 pA Measurement conditions: Dispersion on a copper mesh-supported carbon film was used to perform high-resolution transmission electron microscope (HR-TEM) observation. The results are shown in Figs. 2 to 4. In Figs. 2 to 4, the scale bar indicates a size of 10 nm.

[0078] As shown in the HR-TEM image in Figure 2, no crystal lattice fringes originating from crystalline domains were observed in the powder of Example 1. This indicates that the particles of Example 1 have high internal uniformity.

[0079] As shown in the HR-TEM image in Figure 3, crystal lattice fringes originating from crystalline domains were clearly observed in the powder of Comparative Example 1.

[0080] Furthermore, the HR-TEM observation image shown in Figure 4 clearly shows crystal lattice fringes (dark areas in the figure) in the powder of Comparative Example 2, and it was confirmed that minute crystalline domains shown in these crystal lattice fringes are diffused within the particles. From this, it was found that the internal uniformity of the particles in Comparative Example 2 was not high. When crystalline vanadium sulfide with low internal uniformity is used as an electrode active material, the cycle characteristics tend to deteriorate because the crystalline domains expand after a certain number of charge-discharge cycles, causing delamination at the interfaces between crystalline domains.

[0081] [Test Example 3: Particle Size Measurement] Regarding the powder obtained in Example 1, Measurement device: AEROTRAC II (Microtrac bell) Measurement conditions: Discharge pressure 0.3 MPa Particle size measurements were performed using [a specific method / tool]. The particle size distribution (histogram) is shown in Figure 5.

[0082] As shown in Figure 5, the average particle size D50 of the vanadium sulfide powder obtained in Example 1 was 5.8 μm. The average particle size D50 of the vanadium sulfide powder obtained in Comparative Example 1 was 8.0 μm, and the average particle size D50 of the vanadium sulfide powder obtained in Comparative Example 2 was 2.7 μm.

[0083] [Test Example 4: Charge / Discharge Test] Next, using the vanadium sulfide powder obtained in Example 1 and Comparative Example 2 as the positive electrode active material, a test electrochemical cell (lithium secondary battery) was prepared using the following method. Constant current charge-discharge measurements were performed at 25°C and 60°C with a current density of 10 mA / g (charge-discharge rate: 0.01 C), a voltage in the range of 1.5 to 3.0 V, and a cycle interval of 10 minutes.

[0084] For the preparation of the test electrochemical cell, the obtained vanadium polysulfide was used as the positive electrode active material, lithium indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte to assemble an all-solid-state lithium-ion secondary battery, and charge-discharge tests were performed. For the positive electrode, the vanadium sulfide powder, argyrodite-type sulfide-based solid electrolyte, and acetylene black were mixed in a mass ratio of 80:18:2 to be used as the positive electrode composite material, and a 10 mm diameter pellet battery was fabricated by pressure molding the positive electrode composite material / argyrodite-type sulfide-based solid electrolyte / lithium indium foil.

[0085] The results are shown in Figure 6. The results show that when using the vanadium polysulfide powder obtained in Example 1, a high discharge ratio capacity (25°C: 461 mAh / g, 60°C: 769 mAh / g) was obtained in tests using all-solid-state battery cells. On the other hand, when using the powder obtained in Comparative Example 2, the discharge ratio capacity was lower compared to Example 1 (25°C: 317 mAh / g, 60°C: 649 mAh / g).

Claims

1. Low-crystalline vanadium sulfide, It contains vanadium and sulfur as constituent elements, The composition ratio (S / V) of vanadium to sulfur is 3.5 or more in molar ratio. The average particle size D50 is 10 μm or less. In the X-ray diffraction pattern using CuKα rays, there is no peak at the diffraction angle 2θ = 53.0° within a tolerance of ±1.0°. Low-crystalline vanadium sulfide.

2. General formula (1): VS ｘ (1) [In the equation, x is 3.5 or greater.] A low-crystalline vanadium sulfide according to claim 1, having the composition represented by .

3. VS ４ A low-crystallinity vanadium sulfide according to claim 1, having a type crystal structure.

4. In the aforementioned X-ray diffraction pattern, peaks are found at 2θ = 15.8° and 17.0° within an acceptable range of ±1.0°. The low-crystallinity vanadium sulfide according to claim 1, wherein the crystallite sizes determined from the full width at half maximum of the peaks at 2θ = 15.8° and 17.0° are 15 nm or less.

5. In the aforementioned X-ray diffraction pattern, within a tolerance range of ±1.0°, It does not have peaks at 2θ = 24.4° and 33.0°, or The area of ​​each peak having a maximum at 2θ = 24.4° and 33.0° is 20% or less of the smaller of the areas of the two peaks having a maximum at 2θ = 15.8° and 17.0°. The low-crystalline vanadium sulfide according to claim 4.

6. A method for producing a low-crystallinity vanadium sulfide according to any one of claims 1 to 5, A hydrothermal treatment process in which a raw material containing vanadate and sulfur-containing compounds is hydrothermally treated at 100°C to 160°C in the absence of a support. A manufacturing method that includes the following features.

7. The manufacturing method according to claim 6, wherein the amount of the sulfur-containing compound used is 3.5 moles to 5.0 moles per mole of the vanadate salt.

8. An electrode active material for a lithium-ion secondary battery comprising a low-crystallinity vanadium sulfide according to any one of claims 1 to 5.

9. An electrode for a lithium-ion secondary battery comprising the electrode active material for a lithium-ion secondary battery as described in claim 8.

10. A lithium-ion secondary battery comprising the electrodes for a lithium-ion secondary battery as described in claim 9.

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