JP2014093210A - Positive electrode active material for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a material having high charge/discharge capacity useful as a positive electrode active material for lithium secondary batteries, such as metal lithium secondary batteries and lithium ion secondary batteries, high in conductivity, also good in cycle characteristics, and having excellent charge/discharge performance.SOLUTION: An amorphous titanium polysulfide satisfying the following requirements (1)-(3) has excellent charge/discharge performance as a positive electrode active material for lithium secondary batteries: (1) comprising fine powder of amorphous titanium polysulfide having an average composition represented by a composition formula TiS(where, 2<n<10 is satisfied); (2) having diffraction peaks at at least two sites including a position where a diffraction angle 2θ is 34±1° among positions where the diffraction angle 2θ is 15.5±1°, 34±1°, 44±1° and 54±1°, in an X-ray diffraction chart using a CuKα line; and (3) having a half-value width of the diffraction peak at 2θ=34±1° in the range of 0.3-2.5°.

Description

  The present invention relates to a positive electrode active material for a lithium secondary battery, a manufacturing method thereof, and a lithium secondary battery.

  Secondary batteries (especially lithium batteries) are increasingly required to have higher capacities due to the high performance of portable electronic devices and hybrid vehicles in recent years. In current lithium secondary batteries, the capacity of the positive electrode is insufficient compared to that of the negative electrode, and the capacity of a lithium nickelate material, which is said to be relatively high, is about 190 to 220 mAh / g.

  On the other hand, sulfur has a high theoretical capacity of about 1670 mAh / g and is expected to be used as a positive electrode material. However, there is a problem that it has low electronic conductivity and elutes into an organic electrolyte as lithium polysulfide during charge and discharge. In addition, a technique for suppressing elution into the organic electrolyte is indispensable.

  Metal sulfides have electronic conductivity and little elution into organic electrolytes, but have a lower theoretical capacity than sulfur and are reversible due to large structural changes associated with Li insertion / extraction during charge / discharge. There is a problem that is low. In order to achieve high capacity of metal sulfides, it is necessary to increase the sulfur content. However, in crystalline metal sulfides, the site where Li is inserted during discharge is defined by the crystal space group, and the maximum capacity is Since this is determined, it is difficult to exceed this maximum capacity value.

For example, among the titanium sulfide compounds among the metal sulfides, titanium disulfide (TiS ₂ ) and trifluidized titanium (TiS ₃ ) have been studied as crystalline titanium sulfides, 240 and 350 mAh / g, respectively. Although it has been reported that a discharge capacity of a certain degree is exhibited (see Non-Patent Document 1 below), a further increase in capacity is desired.

On the other hand, as an amorphous titanium sulfide compound, there has been a report example in which a TiS _x (0.7 ≦ x ≦ 9) thin film was fabricated using a pulsed laser deposition method (PLD method) and charge / discharge was performed in an all-solid-state cell. Yes (see Non-Patent Document 2 below). In addition, amorphous TiO _y S _z (2.2 ≦ (y + z) ≦ 3.4, 0.4 ≦ y ≦ 1.6, 1.5 ≦ z ≦ 2.8) thin film formed by rf sputtering was used as an electrode, and an organic electrolyte was used. The charge / discharge test results in the cell have been reported. For example, with regard to TiO _0.6 S _2.8 titanium, it has been reported that a capacity of 1147 mAh g ⁻¹ can be obtained by discharging up to 0.5 V (see Non-Patent Document 1 below). Furthermore, it has been reported that a high capacity (about 400 mAh · g ⁻¹ ) was obtained when an amorphous body of TiS ₃ was prepared and used as an electrode in an all-solid-state cell (non-discussed below) (See Patent Document 3).

  There have been reports on amorphous titanium sulfide compounds as described above, but all are thin-film titanium sulfide compounds formed by a vapor phase method, which are difficult to increase in size and are limited to thin film batteries. The

In addition to the charge / discharge capacity, the conductivity of the electrode is important in order to exhibit sufficient charge / discharge characteristics as an electrode material, and it is desirable that the conductivity at room temperature is about 10 ⁻⁴ S / cm or more. It is. However, when the sulfur content is increased for the purpose of increasing the capacity of the titanium sulfide compound, there is a problem that the conductivity is significantly lowered. In this case, since the high-speed charge / discharge characteristics are lacking, it is desirable to make the particles fine or thin. However, as described above, it is difficult to increase the size of the thin-film electrode, and there is a problem that the application is limited.

M.H.Lindic et al, Solid State Ionics, 176 (2005) 1529-1537. T. Matsuyama et al, J. Mater. Sci. 47 (2012) 6601-6606. A. Hayashi et al, Chem. Lett., 41 (9) (2012) 886-888.

  The present invention has been made in view of the current state of the prior art described above, and its main purpose is useful as a positive electrode active material for lithium secondary batteries such as metal lithium secondary batteries and lithium ion secondary batteries. An object of the present invention is to provide a material having a high charge / discharge capacity, high conductivity, good cycle characteristics, and excellent charge / discharge performance.

The present inventor has intensively studied to achieve the above-described object. As a result, titanium disulfide (TiS ₂ ) having crystallinity and sulfur are used as raw materials, and these are mixed and pulverized by a mechanical milling method, whereby titanium disulfide is made amorphous and the content ratio of sulfur is high. It has been found that an amorphous titanium polysulfide compound can be obtained. The product obtained by this method is obtained by subjecting the product to mechanical milling until a small amount of fine crystals of titanium disulfide (TiS ₂ ) remain without proceeding to complete amorphization. The product is in a state where fine crystals of titanium disulfide are present in the amorphous titanium polysulfide, which contributes to improvement of electron conductivity, ion conductivity, etc., and the positive electrode active material of the lithium secondary battery It was found that when used as, it exhibits excellent charge / discharge performance. The present invention has been completed as a result of further research based on such knowledge.

That is, this invention provides the following active material for lithium secondary batteries, its manufacturing method, and a lithium secondary battery.
Item 1. A positive electrode active material for a lithium secondary battery, which satisfies the following requirements (1) to (3):
(1) Composition formula: TiS _n (wherein 2 <n <10) and consists of fine powder of amorphous titanium polysulfide having an average composition,
(2) In the X-ray diffraction diagram by CuKα ray, the diffraction angle 2θ includes at least 34 ± 1 ° within the positions of 15.5 ± 1 °, 34 ± 1 °, 44 ± 1 °, and 54 ± 1 °. Has diffraction peaks at two locations,
(3) The half width of the diffraction peak at 2θ = 34 ± 1 ° is in the range of 0.3 to 2.5 °.
Item 2. The positive electrode active material for lithium secondary batteries according to item 1, wherein crystalline TiS ₂ and sulfur are used as raw materials, and mixed and pulverized by mechanical milling to react TiS ₂ and sulfur. A method for producing a substance.
Item 3. The positive electrode for lithium secondary batteries containing the positive electrode active material for lithium secondary batteries of said claim | item 1.
Item 4. A lithium secondary battery comprising the positive electrode for a lithium secondary battery according to Item 3 as a constituent element.
Item 5. Item 5. The lithium secondary battery secondary battery according to Item 4, which is a nonaqueous electrolyte secondary battery or an all solid state secondary battery.

  Hereinafter, first, titanium polysulfide, which is an effective component of the positive electrode active material for a secondary battery of the present invention, will be specifically described.

Titanium polysulfide The positive electrode active material for a rechargeable lithium battery of the present invention is an amorphous multi-material having an average composition represented by the composition formula: TiS _n (where 2 <n <10). It consists of a fine powder of titanium sulfide, and a diffraction peak based on a TiS ₂ crystal is observed within a diffraction angle 2θ = 10 ° to 60 ° in an X-ray diffraction diagram by CuKα rays. Specifically, the diffraction angle 2θ = 15.5 ± 1 ° based on the (001) plane of the TiS ₂ crystal, the diffraction peak 2θ = 34 ± 1 ° based on the (011) plane, and the (102) plane Among the diffraction peak of 2θ = 44 ± 1 ° based on the diffraction peak and the diffraction peak of 2θ = 54 ± 1 ° based on the (110) plane, diffraction peaks are observed in at least two places including the diffraction peak of 2θ = 34 ± 1 °. The half width of the diffraction peak at 2θ = 34 ± 1 ° is in the range of 0.3 ° to 2.5 °.

Compared with the fact that the half width of the diffraction peak of 2θ = 34 ± 1 ° of TiS _{2 having} good crystallinity is about 0.2 °, 2θ in titanium polysulfide which is an active ingredient of the positive electrode active material of the present invention. The diffraction peak at = 34 ± 1 ° is a broad peak with a very wide half-value width. This indicates that TiS ₂ contained in the positive electrode active material of the present invention is very fine and has low crystallinity.

In the present invention, the full width at half maximum of the X-ray diffraction peak is determined by a powder X-ray diffraction measurement method, and an example of measurement conditions is as follows.
X-ray source: CuKα 5kV-300mA
Measurement conditions: 2θ = 10 to 60 °, 0.02 ° step, scanning speed 10 ° / min Further, titanium polysulfide which is an active ingredient of the positive electrode active material for lithium secondary battery of the present invention is in an amorphous state. There are no other diffraction peaks based on titanium sulfide other than the X-ray diffraction peaks based on TiS ₂ described above.

In addition, when manufacturing titanium polysulfide by the mechanical milling method mentioned later, the sulfur used as a raw material forms amorphous polysulfide by reaction with TiS ₂ and X-ray diffraction based on sulfur. When no peak is observed or there is an X-ray diffraction peak based on sulfur, the diffraction intensity at the diffraction angle (2θ) at which the sulfur used as the raw material exhibits the maximum intensity is the diffraction intensity of sulfur as the raw material Of 1/5 or less, preferably 1/10 or less.

  For this reason, although the positive electrode active material of the present invention is titanium polysulfide having a high sulfur ratio as an average composition, sulfur hardly exists by itself, and is bonded to titanium in an amorphous state. Polysulfide is formed.

Cathode active material for a lithium secondary battery of the present invention having the features described above, the composition formula: in TiS _n, but n is one having an average composition falls within the range of 2 <n <10, fine of TiS ₂ It only has a broad diffraction peak based on crystals, no other diffraction peak based on titanium sulfide is observed, and as described above, almost no diffraction peak based on sulfur is observed. For this reason, the positive electrode active material for a lithium secondary battery of the present invention is considered to be in a state in which a small amount of refined TiS ₂ crystals are present, mainly composed of amorphous titanium polysulfide fine particles. Incidentally, the above-mentioned composition formula: in TiS _n, the value of n is preferably 2.5 ≦ n ≦ 8, more preferably 3 ≦ n ≦ 6, more preferably from 3 ≦ n ≦ 5.

  In addition, in this-application specification, the average composition of titanium polysulfide shows the element ratio of the titanium and sulfur which comprise the whole titanium polysulfide.

  The positive electrode active material for a lithium secondary battery of the present invention contains titanium polysulfide that satisfies the above-mentioned conditions as an active ingredient, but other impurities may be used as long as the performance of the titanium polysulfide is not impaired. It may be included. Examples of such impurities include metals such as transition metals and typical metals that may be mixed into the raw material, oxygen that may be mixed during the raw material and production, and the like. About the quantity of these impurities, what is necessary is just the range which does not inhibit the performance of the above-mentioned titanium polysulfide compound, usually, with respect to 100 parts by weight of the total amount of titanium and sulfur in titanium polysulfide satisfying the above-mentioned conditions, The amount is preferably about 10 parts by weight or less, more preferably about 5 parts by weight or less, and still more preferably 3 parts by weight or less.

Method for producing titanium polysulfide
Titanium polysulfide, which is an active ingredient of the positive electrode active material for a lithium secondary battery of the present invention, uses crystalline TiS ₂ and sulfur as raw materials, and is mixed and pulverized by mechanical milling to obtain TiS ₂ and sulfur. It can be obtained by reacting.

The mechanical milling method is a method of grinding and mixing raw materials while applying mechanical energy. According to this method, TiS ₂ and sulfur are mixed by applying mechanical impact and friction to the raw materials. Is vigorously contacted and refined to cause a reaction of raw materials. For this reason, it is possible to react the raw materials without heating to a high temperature, and it is possible to obtain amorphous titanium polysulfide without crystallization.

  As the mechanical milling method, specifically, for example, mixed pulverization may be performed using a mechanical pulverizer such as a ball mill, a rod mill, a vibration mill, a disk mill, a hammer mill, a jet mill, or a VIS mill.

TiS ₂ used as a raw material is not particularly limited, and any commercially available TiS ₂ can be used. In particular, it is preferable to use a high-purity one. Further, since TiS ₂ is mixed and pulverized by a mechanical milling method, there is no limitation on the particle size of TiS ₂ to be used, and usually commercially available powdered TiS ₂ may be used.

  The sulfur used as a raw material is not particularly limited, and any crystalline sulfur can be used as long as it is solid at normal temperature and pressure.

TiS ₂ For the ratio of sulfur may be such that the element ratio and the same ratio of titanium and sulfur in polysulfide titanium of interest.

  Regarding the temperature at the time of mechanical milling, if it is too high, volatilization of sulfur is likely to occur, and further, crystallization of the product proceeds, and it is difficult to form a polysulfide having a high target sulfur content ratio. Become. For this reason, it is preferable to perform mechanical milling at a temperature of about 200 ° C. or less.

  The time for mechanical milling is not particularly limited. In X-ray diffraction, the above conditions, that is, the half width of the diffraction peak at 2θ = 34 ± 1 ° is within the range of 0.3 to 2.5 °, and sulfur The mechanical milling process may be performed until a diffraction peak based on the above is hardly observed.

  The target titanium polysulfide can be obtained as a fine powder by the mechanical milling process described above. The resulting titanium polysulfide is a fine powder having an average particle size of about 1 to 10 μm, preferably about 3 to 5 μm.

In the present specification, the average particle diameter is a median diameter (d ₅₀ ) determined by a dry laser diffraction / scattering method.

Use of titanium polysulfide Titanium polysulfide obtained by the above-described method has a high charge / discharge capacity because it is an amorphous polysulfide having an element ratio of S to Ti exceeding 2 as an average composition. It becomes. In addition, according to X-ray diffraction, only a broad diffraction peak of TiS ₂ is recognized, so that it can be determined that fine crystals of TiS ₂ exist in a miniaturized state. This TiS ₂ microcrystal can insert and desorb lithium ions, acts as a positive electrode active material, and has good electronic conductivity and ionic conductivity. Can be improved. Moreover, when TiS ₂ microcrystals are refined by a mechanical milling method, it is considered that they are present in a state of being incorporated into primary particles or secondary particles of amorphous titanium polysulfide. Electron conductivity and ion conductivity can be imparted to the inside, and the inside of the titanium polysulfide is effectively utilized to have a high charge / discharge capacity.

  Titanium polysulfide having such characteristics is useful as a positive electrode active material for lithium secondary batteries such as metal lithium secondary batteries and lithium ion secondary batteries. The lithium secondary battery in which the positive electrode active material of the present invention can be used effectively may be a non-aqueous electrolyte lithium secondary battery using a non-aqueous solvent based electrolyte as the electrolyte, or a lithium ion conductive solid electrolyte It may be an all solid-state lithium secondary battery.

  The structures of the nonaqueous electrolyte lithium secondary battery and the all solid lithium secondary battery can be the same as those of a known lithium secondary battery except that the positive electrode active material of the present invention is used.

  For example, the basic structure of the nonaqueous electrolyte lithium secondary battery may be the same as that of a known nonaqueous electrolyte lithium secondary battery except that the above-described titanium polysulfide is used as the positive electrode active material.

  For the positive electrode, the above-described titanium polysulfide may be used as a positive electrode active material, and a positive electrode mixture containing a conductive agent, a binder, and the like may be supported on a positive electrode current collector such as Al, Ni, and stainless steel. As the conductive agent, for example, carbon materials such as graphite, cokes, and carbon black can be used.

  As the negative electrode, for example, a lithium metal, a lithium alloy, or the like can be used in a metal lithium secondary battery, and a material that can be doped / undoped with lithium ions can be used as an active material in a lithium ion secondary battery. . These negative electrode active materials may be supported on a negative electrode current collector made of Al, Cu, Ni, stainless steel or the like using a conductive agent, a binder, or the like, if necessary.

  The separator is made of, for example, a polyolefin resin such as polyethylene or polypropylene, a fluororesin, nylon, or an aromatic aramid, and a material such as a porous film, a nonwoven fabric, or a woven fabric can be used.

  As the solvent for the non-aqueous electrolyte, known solvents can be used as solvents for non-aqueous solvent secondary batteries such as carbonates, ethers, nitriles, and sulfur-containing compounds.

  The all solid lithium secondary battery may have the same structure as a known all solid lithium secondary battery except that the positive electrode active material of the present invention is used.

  In this case, as the electrolyte, for example, a polymer solid electrolyte such as a polyethylene oxide polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain, a sulfide solid electrolyte, An oxide-based solid electrolyte or the like can be used.

  For the positive electrode of the all solid-state lithium secondary battery, for example, the above-described titanium polysulfide is used as a positive electrode active material, and a positive electrode mixture containing a solid electrolyte in addition to a conductive agent and a binder is used, such as Al, Ni, and stainless steel. What is necessary is just to carry | support to a positive electrode electrical power collector. As for the conductive agent, for example, carbon materials such as graphite, cokes, and carbon black can be used as in the case of the non-aqueous solvent secondary battery.

  The shapes of the nonaqueous electrolyte lithium secondary battery and the all solid-state lithium secondary battery are not particularly limited, and may be any of a cylindrical shape, a rectangular shape, and the like.

The positive electrode active material for a lithium secondary battery of the present invention is composed of titanium polysulfide having an element ratio of S to Ti exceeding 2 and is a polysulfide having a high element ratio of sulfur. It will have. In addition, since TiS ₂ microcrystals having good electron conductivity and ionic conductivity are present in the state of being incorporated in the primary particles or secondary particles of amorphous titanium polysulfide, titanium polysulfide Can be effectively utilized up to the inside of the battery, and has a high charge / discharge capacity and energy density. Furthermore, the cycle characteristics are also good.

  For this reason, the positive electrode active material for lithium secondary batteries of the present invention is useful as a positive electrode active material for lithium secondary batteries such as metal lithium secondary batteries and lithium ion secondary batteries, and uses a non-aqueous solvent electrolyte. It can be effectively used as a positive electrode active material for water electrolyte lithium secondary batteries and all solid lithium secondary batteries using solid electrolytes.

It is drawing which shows the X-ray-diffraction pattern of the fine powder obtained in Examples 1, 2 and Comparative Examples 1, 2. It is drawing which shows the charging / discharging curve of the all-solid-type lithium secondary battery which uses the microparticles | fine-particles obtained in Example 1 as a positive electrode active material. It is drawing which shows the charging / discharging curve of the all-solid-type lithium secondary battery which uses the microparticles | fine-particles obtained by the comparative example 1 as a positive electrode active material. It is drawing which shows the charging / discharging curve of the all-solid-type lithium secondary battery which uses the microparticles | fine-particles obtained in Example 2 as a positive electrode active material. It is drawing which shows the charging / discharging curve of the all-solid-state lithium secondary battery which uses the microparticles | fine-particles obtained by the comparative example 2 as a positive electrode active material.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
Commercially available titanium disulfide (TiS ₂ ) powder and commercially available sulfur (S ₈ ) powder are weighed and mixed so that the element ratio is Ti: S = 1: 3, and then about 400 zirconia balls with a diameter of 4 mm Using a 45 mL container containing slag, mechanical milling was performed at 360 rpm for 1 hour using a ball mill (Fritche P7).

FIG. 1 shows an X-ray diffraction pattern obtained by XRD measurement using CuKα rays for the obtained fine powder. In the X-ray diffraction diagram shown in FIG. 1, diffraction peaks having a small intensity and a large half-value width were confirmed, indicating the presence of TiS ₂ nanocrystals at positions where 2θ was about 15.5, 34.2, 44, and 54 °. The full width at half maximum of the X-ray diffraction peak observed at 2θ = 34.2 ° was 0.40. The average particle diameter d ₅₀ obtained by the dry laser diffraction / scattering method was 3.0 μm, and the maximum particle diameter was about 20 μm.

FIG. 1 further shows a single X-ray diffraction diagram of titanium disulfide and sulfur used as raw materials, and an X-ray diffraction diagram of a mixture of titanium disulfide and sulfur. As is clear from FIG. 1, the TiS ₂ used as a raw material has a strong and sharp diffraction pattern at each position where 2θ is 15.6 °, 34.2 °, 44.2 °, 53.8 °, and 57.7 °. The value range was 0.22 °, and it was found that the TiS ₂ crystal had higher crystallinity (large crystallite size) than the fine powder obtained by the mechanical milling method.

  Further, in the X-ray diffraction diagram of the raw material sulfur, a strong diffraction peak was present in the vicinity of 2θ = 23 °, but the diffraction peak based on sulfur disappeared in the fine powder obtained by the mechanical milling method.

From this result, the fine powder obtained by the mechanical milling process is an amorphous titanium polysulfide represented by an average composition: TiS ₃ and is in a state where fine crystals of TiS ₂ are mixed. I was able to confirm.

Comparative Example 1
TiS ₂ powder and S ₈ powder were weighed and mixed so that the element ratio was Ti: S = 1: 3, and then subjected to mechanical milling for 40 hours in the same ball mill apparatus as in Example 1.

The X-ray diffraction pattern of the fine powder obtained is shown in FIG. In the X-ray diffraction pattern shown in FIG. 1, no crystal peak was confirmed, and it was found that the TiS ₃ was amorphous TiS _{3 in} which no TiS ₂ crystal was present.

Charge / discharge test 1
Using each fine powder obtained in Example 1 and Comparative Example 1 described above as a positive electrode active material, an all solid-state lithium secondary battery for testing was prepared by the following method. At a current density of 14 mA / g, A charge / discharge test was performed at the start of charging by constant current measurement at a cutoff of 1.3-2.4V.

As a method for producing an all-solid-state lithium secondary battery for testing, first, as a positive electrode material, fine powder (positive electrode active material) obtained in Example 1 or Comparative Example 1, carbon black, and sulfide-based solid electrolyte (80Li ₂ S · 20P ₂ S ₅ ) was weighed so that the positive electrode active material: carbon black: sulfide diameter solid electrolyte (weight ratio) = 64: 6: 30 and kneaded in a mortar for 5 minutes. 10 mg of the kneaded product was uniformly filled in a molding machine having a diameter of 10 mm, and further 80 mg of a sulfide-based solid electrolyte (80Li ₂ S · 20P ₂ S ₅ ) was laminated, followed by uniaxial molding at 360 MPa. After that, an indium foil with a thickness of 0.3 mm and a lithium foil with a thickness of 0.2 mm were pasted on the sulfide-based solid electrolyte side as a negative electrode, and then uniaxially molded at 120 MPa. A battery was obtained. Stainless steel was used as the current collector for both the positive electrode and the negative electrode.

  FIG. 2 shows a charge / discharge curve when the fine powder obtained in Example 1 is used as the positive electrode active material, and FIG. 3 shows a charge / discharge curve when the fine powder obtained in Comparative Example 1 is used as the positive electrode active material.

  When the fine powder obtained in Comparative Example 1 was used as the positive electrode active material, the initial discharge capacity was about 341 mAh / g and the initial charge capacity was 217 mAh / g, whereas it was obtained in Example 1. When the fine powder was used as the positive electrode active material, the initial discharge capacity was about 420 mAh / g and the initial charge capacity was 350 mAh / g, indicating a high charge / discharge capacity.

Example 2
Commercially available titanium disulfide (TiS ₂ ) powder and commercially available sulfur (S ₈ ) powder are weighed and mixed so that the element ratio is Ti: S = 1: 4, and then about 400 zirconia balls with a diameter of 4 mm Using a 45 mL container containing slag, mechanical milling was performed at 360 rpm for 1 hour using a ball mill (Fritche P7).

FIG. 1 shows an X-ray diffraction pattern obtained by XRD measurement using CuKα rays for the obtained fine powder. In the X-ray diffraction pattern shown in FIG. 1, diffraction peaks having a small intensity and a large half-value width were confirmed, indicating the presence of TiS ₂ nanocrystals at angular positions where 2θ was about 15.5, 34.2, 44, and 54 °. The diffraction peak indicating the presence of S ₈ disappeared. The half width of the X-ray diffraction peak observed at 2θ = 34.2 ° was 2.0 °. The average particle diameter d ₅₀ obtained by the dry laser diffraction / scattering method was 4.9 μm, and the maximum particle diameter was about 30 μm.

From this result, the fine powder obtained by the mechanical milling process is a polysulfide in an amorphous state represented by an average composition: TiS ₄ and is in a state where fine crystals of TiS ₂ are present. I was able to confirm.

Comparative Example 2
TiS ₂ powder and S ₈ powder were weighed and mixed so that the element ratio was Ti: S = 1: 4, and then subjected to mechanical milling for 40 hours with a ball mill apparatus.

The X-ray diffraction pattern of the fine powder obtained is shown in FIG. In the X-ray diffraction diagram of FIG. 1, no crystal peak was confirmed for the sample of Comparative Example 2, and it was found that the sample was amorphous TiS _{4 in} which no TiS ₂ crystal was present.

Charge / discharge test 2
An all-solid-state lithium secondary battery for testing was prepared and charged / discharged in the same manner as in the charge / discharge test 1 except that each fine powder obtained in Example 2 and Comparative Example 2 was used as the positive electrode active material. Went.

  FIG. 4 shows a charge / discharge curve when the fine powder obtained in Example 2 is used as the positive electrode active material, and FIG. 5 shows a charge / discharge curve when the fine powder obtained in Comparative Example 1 is used as the positive electrode active material.

  When the fine powder obtained in Comparative Example 2 was used as the positive electrode active material, the initial discharge capacity was about 331 mAh / g and the initial charge capacity was 154 mAh / g, whereas the fine powder obtained in Example 2 was used. When the powder was used as the positive electrode active material, the initial discharge capacity was about 485 mAh / g and the initial charge capacity was 350 mAh / g, indicating a high charge / discharge capacity.

Claims (5)

A positive electrode active material for a lithium secondary battery, which satisfies the following requirements (1) to (3):
(1) Composition formula: TiS _n (wherein 2 <n <10) and consists of fine powder of amorphous titanium polysulfide having an average composition,
(2) In the X-ray diffraction diagram by CuKα ray, the diffraction angle 2θ includes at least 34 ± 1 ° within the positions of 15.5 ± 1 °, 34 ± 1 °, 44 ± 1 °, and 54 ± 1 °. Has diffraction peaks at two locations,
(3) The half width of the diffraction peak at 2θ = 34 ± 1 ° is in the range of 0.3 to 2.5 °. _{2. The} positive electrode active material for a lithium secondary battery according to claim 1, wherein crystalline TiS ₂ and sulfur are used as raw materials and mixed and pulverized by a mechanical milling method to react TiS ₂ and sulfur. A method for producing a substance. The positive electrode for lithium secondary batteries containing the positive electrode active material for lithium secondary batteries of Claim 1. A lithium secondary battery comprising the positive electrode for a lithium secondary battery according to claim 3 as a constituent element. The lithium secondary battery according to claim 4, which is a nonaqueous electrolyte secondary battery or an all-solid-type secondary battery.

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