JP7699335B2 - Positive electrode active material, positive electrode material, secondary battery including the positive electrode material, and method for manufacturing the positive electrode active material - Google Patents

Description

The present invention relates to a positive electrode active material, a positive electrode material, a secondary battery including the positive electrode material, and a method for producing the positive electrode active material.

Demands for next-generation secondary batteries such as lithium-ion batteries are increasing year by year, and in recent years, in addition to high safety, there is also a demand for high energy density and high cycle characteristics. In order to meet these high demands for next-generation secondary batteries, active development efforts have been made to improve various electrode materials, and various high-performance electrode (positive and negative) active material materials have been developed (see, for example, Patent Document 1, etc.).

For example, two-dimensional layered sulfide _MS2 (where M represents a metal atom) has attracted increasing attention as a promising electrode material for energy storage applications due to the weak van der Waals forces between the stacked layers. In this regard, much attention has been paid recently to the design of nanostructures to improve the performance of _MS2 as a battery electrode material.

JP 2020-107614 A

However, _MS2 has low electrical conductivity despite its large interlayer spacing (0.615 nm). For this reason, it is difficult to improve the coulombic efficiency and to increase the capacity, which causes problems in the cycle characteristics of the battery and limits its application in the field of energy storage. From this perspective, there has been a demand for further improvement in the cycle characteristics of electrode materials using _MS2 .

The present invention has been made in view of the above, and aims to provide a positive electrode active material that can provide a secondary battery with excellent cycle characteristics, a positive electrode material containing this positive electrode active material, and a method for producing the positive electrode active material.

As a result of extensive research into achieving the above objective, the inventors discovered that the above objective could be achieved by using sulfides of specific transition metal elements as constituent components, and thus completed the present invention.

That is, the present invention includes, for example, the subject matter described in the following sections.
Item 1
A positive electrode active material comprising a sulfide of a transition metal element M1 and at least one transition metal element M2 other than the transition metal element M1.
Section 2
Item 2. The positive electrode active material according to item 1, wherein the transition metal element M1 is Mo, W or V.
Section 3
Item 3. The positive electrode active material according to item 1 or 2, wherein the transition metal element M2 is at least one selected from the group consisting of Mo, W, and V.
Section 4
Item 4. The positive electrode active material according to any one of items 1 to 3, wherein the sulfide has a part of the transition metal element M1 replaced with the transition metal element M2.
Section 5
Item 5. The positive electrode active material according to any one of items 1 to 4, having a laminated structure.
Item 6
Item 6. A positive electrode material comprising the positive electrode active material according to any one of items 1 to 5.
Section 7
Item 7. A secondary battery comprising the positive electrode material according to item 6.
Section 8
Item 6. A method for producing a positive electrode active material according to any one of items 1 to 5,
A step A of obtaining a product by heat-treating a raw material including a transition metal element M1 source, a transition metal element M2 source, a sulfur source, and a solvent;
A step B of calcining the product obtained in the step A;
The method for producing a positive electrode active material comprises the steps of:

The positive electrode active material of the present invention can provide secondary batteries with excellent cycle characteristics.

1 shows an SEM image of the positive electrode active material (LS-VMS) obtained in Example 1. FIG. 1 shows an SEM image in which element mapping of sulfur (S), molybdenum (Mo), and vanadium (V) elements is performed. 1 shows the results of X-ray diffraction measurement (XRD) of the positive electrode active material (LS-VMS) obtained in Example 1. FIG. 2 is a schematic diagram showing a part of the structure of the positive electrode active material (LS-VMS) obtained in Example 1. 1A shows the results of a constant current charge/discharge test using the battery assembled in Preparation Example 1, and FIG. 1B shows the results of a constant current charge/discharge test using the battery assembled in Preparation Example 2.

The embodiments of the present invention will be described in detail below. In this specification, the expressions "contain" and "include" include the concepts of "contain," "include," "consist essentially of," and "consist only of."

The positive electrode active material of the present invention contains a sulfide of a transition metal element M1 and at least one transition metal element M2 other than the transition metal element M1. The positive electrode active material of the present invention can be used as a positive electrode material for constituting various secondary batteries, and can provide the secondary batteries with excellent cycle characteristics.

The transition metal element M1 can be exemplified by various transition metal elements. In terms of being more likely to provide the secondary battery with excellent cycle characteristics, the transition metal element M1 is preferably Mo, W, or V.

Therefore, the sulfide of the transition metal element M1 can be exemplified by sulfides of various transition metal elements, and is preferably a sulfide of Mo, a sulfide of W, or a sulfide of V, since these tend to provide the secondary battery with better cycle characteristics.

The sulfide of the transition metal element M1 is particularly preferably a sulfide of Mo (for example, MoS ₂ ) in that it is likely to impart particularly excellent cycle characteristics to the secondary battery.

As described above, the positive electrode active material of the present invention further contains a transition metal element M2 different from the transition metal element M1 in addition to the sulfide of the transition metal element M1. The positive electrode active material may contain only one type of transition metal element M2, or may contain two or more types.

The transition metal element M2 can be exemplified by various transition metal elements, and among them, the transition metal element M2 is preferably at least one selected from the group consisting of Mo, W, and V, and more preferably at least V, in that it is likely to provide the secondary battery with superior cycle characteristics.

In the positive electrode active material of the present invention, the state of existence of the transition metal element M2 is not particularly limited. For example, the transition metal element M2 can be present by replacing the transition metal element M1 in the sulfide (i.e., the transition metal element M2 can also be bonded to sulfur). Specifically, the sulfide can have a structure in which a part of the transition metal element M1 is replaced by the transition metal element M2 (i.e., in the positive electrode active material of the present invention, the transition metal element M2 can be present, for example, in the framework of the sulfide of the transition metal element M1). In this case, the positive electrode active material is likely to provide a secondary battery with better cycle characteristics.

The presence of the transition metal element M2 in the framework of the sulfide of the transition metal element M1 can be determined, for example, by various known methods, for example, from the XRD spectrum of the positive electrode active material.

In the positive electrode active material, the ratio of the transition metal element M1 to the transition metal element M2 is not particularly limited. For example, the molar ratio of the transition metal M1 to the transition metal M2 in the positive electrode active material, M1:M2, is preferably 1:0.2 to 1:5, and more preferably 1:1 to 1:4. This allows the positive electrode active material to provide the secondary battery with better cycle characteristics.

In the positive electrode active material, the transition metal element M1, the transition metal element M2, and the sulfur atoms are preferably uniformly dispersed throughout the positive electrode active material. The dispersion state of the transition metal element M1, the transition metal element M2, and the sulfur atoms in the positive electrode active material can be understood, for example, by element mapping based on SEM images.

As long as the positive electrode active material contains the sulfide of the transition metal element M1 and the transition metal element M as essential components, it can contain other components to the extent that the effects of the present invention are not impaired. For example, the positive electrode active material can contain metals other than the transition metal elements M1 and M2, and examples include metal elements that are inevitably contained in the positive electrode active material. When the positive electrode active material contains other metals than the sulfide of the transition metal element M1 and the transition metal element M2, the content ratio of the other metals is 5 mass% or less, preferably 1 mass% or less, and more preferably 0.1 mass% or less, based on the total mass of the sulfide of the transition metal element M1 and the transition metal element M2.

The positive electrode active material is, for example, a solid such as a powder, but the shape is not particularly limited and can be formed into various shapes. In particular, the positive electrode active material preferably has a laminated structure, and more specifically, the positive electrode active material preferably has a so-called layer-by-layer structure in which multiple layers are stacked in layers. This allows the positive electrode active material to provide the secondary battery with even better cycle characteristics. In particular, the layer-by-layer structure can provide a buffer effect against the volume expansion of the positive electrode material during battery charge and discharge, and in addition, it is easy to provide an ion diffusion channel in the electrode material, which is advantageous for rapid ion diffusion. In addition, the positive electrode active material can also form a nanoflower structure.

The method for producing the positive electrode active material is not particularly limited. For example, the positive electrode active material of the present invention can be produced by a production method including steps A and B described below.

2. Method for Producing Positive Electrode Active Material The method for producing a positive electrode active material of the present invention may include, for example, the following steps A and B.
Step A: A step of obtaining a product by heat-treating a raw material including a source of a transition metal element M1, a source of a transition metal element M2, a sulfur source, and a solvent.
Step B: A step of calcining the product obtained in step A.

The manufacturing method including the above-mentioned steps A and B can produce a positive electrode active material, for example, the above-mentioned positive electrode active material of the present invention can be produced.

(Process A)
Step A is a step of heat-treating a raw material including a source of a transition metal element M1, a source of a transition metal element M2, a sulfur source, and a solvent.

The transition metal element M1 can be exemplified by various transition metal elements, and among them, the transition metal element M1 is preferably Mo, W or V, since these tend to provide the secondary battery with better cycle characteristics.

The transition metal element M2 can be exemplified by various transition metal elements, and among them, the transition metal element M2 is preferably at least one selected from the group consisting of Mo, W, and V, and more preferably at least V, in that it is likely to provide the secondary battery with superior cycle characteristics.

The transition metal element M1 source (hereinafter simply referred to as "M1 source") may be the transition metal element M1 alone or a compound containing the transition metal element M1, but is preferably a compound containing the transition metal element M1.

The type of compound containing the transition metal element M1 is not particularly limited, and examples thereof include various inorganic compounds containing the transition metal element M1. Examples of inorganic compounds containing the transition metal element M1 include oxides of the transition metal element M1, compounds containing an oxoanion of the transition metal element M1 (metal acid salts), as well as nitrates, sulfates, chlorides, chlorates, perchlorates, carbonates, hydrogencarbonates, phosphates, and hydrogenphosphates of the transition metal element M1. Among these, it is preferable that the inorganic compound containing the transition metal element M1 is a compound containing an oxoanion of the transition metal element M1.

Specific examples of compounds containing an oxoanion of a transition metal element M1 include vanadates, molybdates, tungstates, etc. The type of salt is not particularly limited, and examples include ammonium salts and alkali metal salts. Examples of vanadates include sodium metavanadate (NaVO ₃ ). Examples of molybdates include sodium molybdate (Na ₂ MoO ₄ ). Examples of tungstates include sodium metavanadate (NaVO ₃ ).

The M1 source can be various organic compounds of the transition metal element M1, such as acetates, oxalates, formates, and succinates of the transition metal element M1.

The source of the transition metal element M2 (hereinafter simply referred to as "M2 source") may be the transition metal element M2 alone or a compound containing the transition metal element M2, but is preferably a compound containing the transition metal element M2.

The type of compound containing the transition metal element M2 is not particularly limited, and examples thereof include various inorganic compounds containing the transition metal element M2. Examples of inorganic compounds containing the transition metal element M2 include oxides of the transition metal element M2, compounds containing oxoanions of the transition metal element M2 (metal acid salts), as well as nitrates, sulfates, chlorides, chlorates, perchlorates, carbonates, hydrogencarbonates, phosphates, and hydrogenphosphates of the transition metal element M2. Among these, it is preferable that the inorganic compound containing the transition metal element M2 is a compound containing an oxoanion of the transition metal element M2.

Specific examples of compounds containing an oxoanion of a transition metal element M2 include vanadates, molybdates, tungstates, and the like. The type of salt is not particularly limited, and examples thereof include ammonium salts and alkali metal salts. Examples of vanadates include sodium metavanadate (NaVO ₃ ). Examples of molybdates include sodium molybdate (Na ₂ MoO ₄ ). Examples of tungstates include sodium metavanadate (NaVO ₃ ).

The M2 source can be various organic compounds of the transition metal element M2, such as acetates, oxalates, formates, and succinates of the transition metal element M1.

The sulfur source used in step A may be elemental sulfur or a compound containing sulfur, but is preferably a compound containing sulfur.

Examples of the sulfur-containing compound include a wide variety of known sulfur compounds, such as thioacetamide (CH ₃ CSNH ₂ ), thiourea (SC(NH ₂ ) ₂ ), cysteine (C ₃ H ₇ NO ₂ S), sodium thiosulfate (Na ₂ S ₂ O ₃ ), ammonium sulfide ((NH ₄ ) ₂ S), sodium sulfide (Na ₂ S), etc. In addition, in the sulfur-containing compound, part of the sulfur element may be replaced with Se and/or Te element.

The sulfur source can be used alone or in combination of two or more types.

The raw materials used in step A include the M1 source, M2 source, and sulfur source as well as a solvent. As the solvent, water can be used, and various organic solvents can also be used. Examples of organic solvents include N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), oleic acid, ethylene glycol, octadecene, and ethylenediamine. The solvent used in step A can also be a mixture of water and an organic solvent.

The solvent may contain various additives. In addition to aqueous ammonia, additives may include dispersion stabilizers and surfactants such as polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB), sodium lauryl sulfonate (SDS), and ethylenediaminetetraacetic acid (EDTA). When the solvent contains an additive, it becomes easier to obtain a positive electrode active material having the layer-by-layer structure described above. When the solvent contains an additive, the content of the additive relative to the total mass of the solvent can be 5% by mass or less, preferably 3% by mass or less, and more preferably 1% by mass or less.

In the raw materials used in step A, the content ratios of the M1 source, M2 source, sulfur source, and solvent are not particularly limited. For example, it is preferable that the raw materials used in step A contain the M1 source and the M2 source such that the molar ratio of the transition metal element M1 to the transition metal element M2, M1:M2, is 1:0.2 to 1:5, preferably 1:1 to 1:4. This allows the resulting positive electrode active material to provide a secondary battery with better cycle characteristics.

In addition, in the raw materials used in step A, the content of the sulfur source is preferably 1 to 30 mol % relative to the total number of moles of the M1 source and the M2 source, more preferably 2 to 15 mol %, and even more preferably 3 to 10 mol %, in that this makes it easier to form a sulfide of the transition metal element M1 and easier to obtain the desired positive electrode active material.

The amount of solvent contained in the raw materials used in step A is not particularly limited. For example, the amount of solvent used in the raw materials can be adjusted so that the concentration of the transition metal element M1 in the raw materials is 1 to 1000 mM, more preferably 10 to 500 mM, and even more preferably 20 to 100 mM.

In step A, the raw materials are heat-treated. This produces a product containing sulfides. One method of heat treatment is, for example, to place the raw materials in a container, seal the container, and heat the container. When the solvent in the raw materials is water, this is known as hydrothermal synthesis.

The temperature inside the container during the heat treatment in step A is not particularly limited and can be, for example, 100 to 400°C, and is preferably 150 to 250°C. The heating time is also not particularly limited and can be appropriately determined depending on the heating temperature, and can be, for example, 6 to 40 hours. The pressure inside the container during the heat treatment can also be set appropriately.

In step A, after the heat treatment is completed, the product can be extracted by an appropriate method. For example, if the product is obtained as a solid, the solid can be separated by a method such as centrifugation, washed, and dried to obtain the product as a solid.

(Process B)
In step B, the product obtained in step A is calcined. By such calcination, the target positive electrode active material is obtained.

In step B, the firing method is not particularly limited, and any known firing method can be widely adopted. For example, the firing temperature can be 200 to 1000°C, preferably 220 to 800°C, more preferably 250 to 600°C, and even more preferably 280 to 500°C.

The firing time may be appropriately selected depending on the firing temperature, and may be, for example, 0.5 to 5 hours. In step 2, the heating rate during firing is not particularly limited and may be appropriately set, for example, 1 to 10°C/min.

The calcination process may be carried out in air or in an inert gas atmosphere. For example, a commercially available heating furnace or other known heating device may be used for the calcination process.

By the calcination treatment in step B, for example, remaining by-products and unreacted raw materials such as sulfur sources are removed, the desired positive electrode active material can be obtained with high purity, and the crystallinity of the resulting positive electrode active material can be increased.

The manufacturing method including the above-mentioned steps A and B makes it possible to easily obtain the positive electrode active material of the present invention through a simple process.

3. Positive electrode material The positive electrode material of the present invention may contain other components as long as it contains the positive electrode active material, for example, known components used in the positive electrode material of secondary batteries (particularly lithium secondary batteries). For example, the positive electrode material of the present invention may contain a conductive assistant and a binder in addition to the positive electrode active material.

The conductive assistant may be, for example, a wide variety of known conductive assistants used to form electrode materials for secondary batteries. Examples of the conductive assistant include various carbon materials, such as natural graphite, artificial graphite, conductive carbon black, graphene, and carbon fibers. Examples of carbon fibers include carbon nanofibers and carbon nanotubes. Other conductive assistants that may be used include metal powders such as copper and nickel, metal fibers, and conductive ceramic materials.

Binder: For example, a wide range of known binders used to form electrode materials for secondary batteries can be used. Examples of binders include various resin materials, specifically polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), polyethylene terephthalate, polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyethylene, polypropylene, etc.

In the positive electrode material, the content ratio of the positive electrode active material is not particularly limited. For example, the positive electrode active material is preferably contained in an amount of 50 to 95 mass%, and more preferably 60 to 90 mass%, of the total mass of the positive electrode active material, conductive additive, and binder contained in the positive electrode material.

The content ratio of the conductive assistant in the positive electrode material is not particularly limited. For example, the conductive assistant is preferably contained in an amount of 3 to 30 mass %, and more preferably 5 to 20 mass %, of the total mass of the positive electrode active material, conductive assistant, and binder contained in the positive electrode material.

The content of the binder in the positive electrode material is not particularly limited. For example, the binder is preferably contained in an amount of 3 to 30 mass %, and more preferably 5 to 20 mass %, based on the total mass of the positive electrode active material, conductive additive, and binder contained in the positive electrode material.

The positive electrode material may consist of only a positive electrode active material, a conductive additive, and a binder, or may contain other components.

The method for preparing the positive electrode material is not particularly limited, and for example, any known method for preparing a positive electrode material can be widely adopted. For example, the positive electrode material can be prepared by mixing the positive electrode active material, the conductive assistant, and the binder in a predetermined ratio by an appropriate method. When preparing the positive electrode material, a solvent can be used to disperse the positive electrode active material, the conductive assistant, and the binder. Examples of the solvent include water and various organic solvents, such as lower alcohol compounds having 1 to 3 carbon atoms and NMP (N-methyl-2-pyrrolidone). When the positive electrode material contains a solvent, it is, for example, in the form of a slurry or a paste.

4. Secondary battery The secondary battery of the present invention is not particularly limited in other configurations as long as it includes the positive electrode material, and may have, for example, the same configuration as a known secondary battery. The type of the secondary battery is not particularly limited, and may be, for example, a lithium ion secondary battery.

The secondary battery may, for example, comprise a positive electrode, a negative electrode, an electrolyte, and a separator. The size and shape of the battery may be determined appropriately depending on the application.

The positive electrode can have a structure in which the positive electrode active material is supported on a metal foil, for example. Examples of metals for forming the metal foil include aluminum, titanium, platinum, molybdenum, stainless steel, and copper. The positive electrode can be produced by a known method, and for example, a positive electrode in which the positive electrode active material is supported on the metal foil can be formed by applying the above-mentioned positive electrode material of the present invention onto the metal foil.

The negative electrode may have a structure in which a negative electrode active material is supported on a metal foil. Examples of the metal foil include aluminum, titanium, platinum, molybdenum, stainless steel, and copper. Examples of the negative electrode active material include metals such as Li, Na, K, Mg, Al, and Zn; graphite and other carbon materials; Si(C)-based, Si(O)-based, or Sn-based alloys or metal oxides; metal sulfides; Li ₄ Ti ₅ O ₁₂ ; and the like. The negative electrode may be prepared by a known method.

In the secondary battery, the type of electrolyte is not particularly limited, and for example, a known electrolyte used in secondary batteries can be used. The electrolyte may be either a solid electrolyte or a liquid electrolyte.

The liquid electrolyte can be a solution in which the electrolyte is dissolved in a solvent.The electrolyte can be various alkali salts, such as _NaPF6 , NaClO4, _NaCF3SO3 , _NaFSI , NaTFSI, LiPF6, _LiClO4 , _LiBF4 , _LiBOB , _LiAsF6 , _LiCF3SO3 , LiTFSI, LiFSI, _KPF6 , _KFSI , KTFSI, _KBF4 , _etc. , and also known magnesium salts, aluminum salts, zinc salts, etc.The solvent can be water, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, propyl acetate, fluoroethylene carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, etc.

Solid electrolytes include inorganic materials such as sulfides and oxides, and polymeric materials such as PEO (polyethylene oxide).

The separator may be a known separator used in secondary batteries, such as polyolefin resins such as polyethylene and polypropylene; polyimide; polyvinyl alcohol; fluororesins such as aminated polyethylene oxide polytetrafluoroethylene; acrylic resins; nylon; aromatic aramid; inorganic glass; ceramics. The separator may be in the form of a porous membrane, nonwoven fabric, woven fabric, or the like. Other examples of the separator include various polymer membranes and inorganic electrolytes. Examples of inorganic electrolytes include LiLaTiO ₃ , Li ₇ La ₃ Zr ₂ O ₁₂ (LLZO), Na ₃ Zr ₂ Si ₂ PO ₁₂ , Na ₁₁ Sn ₂ PS ₁₂ , and Na ₃ PSe ₄ .

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

Example 1
0.234 g of NH ₄ VO ₃ , 0.3629 g of Na ₂ MoO ₄ ·2H ₂ O, 30 mL of distilled water, and 0.25 ml of 28 mass% ammonia water were mixed, 2.402 g of C ₂ H ₅ NS was added thereto, and the mixture was stirred for 1 hour to prepare a raw material.

The obtained raw materials were transferred to a 50 mL autoclave lined with Teflon (registered trademark), the autoclave was sealed, and heated at 220°C for 24 hours (step 1). The product in the autoclave was then collected by centrifugation, washed several times with distilled water and ethanol, and then dried in a vacuum oven at 60°C for 12 hours. Finally, the product was calcined at 300°C for 1 hour in an argon atmosphere to obtain the desired positive electrode active material (step 2). The obtained positive electrode active material was designated "LS-VMS".

(Preparation Example 1)
A positive electrode material was prepared, which consisted of the LS-VMS obtained in Example 1 as the positive electrode active material, superP (conductive carbon black) as the conductive additive, and polyvinylidene fluoride (PVDF) as the binder. In this positive electrode material, the ratio of LS-VMS:superP:PVDF was 8:1:1 (mass ratio). N-methyl-2-pyrrolidone (NMP) was added as a solvent to the positive electrode material, and the mixture was stirred for 12 hours to mix uniformly. The obtained slurry was coated on an Al current collector (aluminum foil), and dried in a vacuum at 120°C for 12 hours to produce a positive electrode. A battery was assembled by a known method using this positive electrode, a negative electrode (lithium metal with copper foil), a liquid electrolyte, and a separator ("Whatman GF/C glass fiber filter paper" provided by Cytiva) impregnated with the liquid electrolyte. The electrolyte was a 1M lithium hexafluorophosphate (LiPF ₆ ) solution, and the solvent of this solution was a mixed solvent of ethylene carbonate and ethyl methyl carbonate (volume ratio 1:1).

(Preparation Example 2)
A battery was assembled in the same manner as in Preparation Example 1, except that the positive electrode active material was changed from LS-VMS to commercially available LiFePO ₄ (HEOLOTECHNALOGYGROUP Co. Ltd., China).

Figure 1 shows an SEM image of the LS-VMS obtained in Example 1. From this SEM image, it was found that the LS-VMS obtained in Example 1 formed a layer-by-layer structure, i.e., a layer-by-layer structure.

Figure 2 shows element mapping images of sulfur (S), molybdenum (Mo), and vanadium (V) in the SEM image of Figure 1. Figure 2(B) is an image showing the results of mapping the area within the frame in Figure 2(A) with S, Mo, and V. From the mapping results in Figure 2, it was confirmed that in the LS-VMS obtained in Example 1, S, Mo, and V are uniformly distributed throughout the entire positive electrode active material.

Figure 3 shows the results of X-ray diffraction measurement (XRD) of the LS-VMS obtained in Example 1. For the X-ray diffraction measurement, a "SmartLab" manufactured by Rigaku Corporation was used, and the measurement was performed using a Cu-Kα (λ = 1.540 Å) radiation source in the range of 2θ = 10 to 100°.

As can be seen from the XRD spectrum in Figure 3, diffraction peaks were clearly observed at 2θ = 14.6°, 2θ = 32.9°, 2θ = 39.5° and 2θ = 58.3°. These diffraction peaks belonged to the (002), (100), (103) and (110) planes of _2H -MoS2, respectively (see JCPDS No. 37-1492). On the other hand, it was found that no structure indicating _VS2 was observed in the XRD spectrum in Figure 3.

As a result, as shown in FIG. 4, it is found that the LS-VMS obtained in Example 1 has a structure in which vanadium element (V) is embedded in the framework of molybdenum disulfide (MoS ₂ ).

Figure 5 (A) shows the results of a constant current charge/discharge test using the battery assembled in Example 1, and Figure 5 (B) shows the results of a constant current charge/discharge test using the battery assembled in Example 2. These measurements were taken using a LAND battery test system "CT2001A" (Wuhan LAND electronics Co., Ltd. China). Here, the measurement temperature was 30°C, and the applied voltage was 1.5 to 3.5 V.

As shown in Fig. 5A, the battery of Preparation Example 1 having the LS-VMS obtained in Example 1 as the positive electrode active material has a Coulombic efficiency of about 100% at a high current density of 1 Ag ^-1 after 1000 cycles, and can provide a specific capacity of 118 mAhg ^-1 after 1000 cycles. In contrast, as shown in Fig. 5B, the battery of Preparation Example 2 having commercially available _LiFePO4 as the positive electrode active material has a Coulombic efficiency of only about 96.7% at a low current density of 0.17 Ag ^-1 after 1000 cycles, and also has a specific capacity of only 109 mAhg ^-1 after 1000 cycles.

From the above, it was found that the secondary battery using the LS-VMS obtained in Example 1 as the positive electrode active material has excellent cycle characteristics.

Claims (3)

A method for producing a positive electrode active material containing a sulfide of a transition metal element M1 and at least one transition metal element M2 other than the transition metal element M1, comprising:
The transition metal element M1 is Mo, W or V;
The transition metal element M2 is at least one selected from the group consisting of Mo, W, and V,
The transition metal element M1 and the transition metal element M2 are different from each other,
The positive electrode active material forms a laminated structure,
A step A of obtaining a product by heat-treating a raw material including a transition metal element M1 source, a transition metal element M2 source, a sulfur source, and a solvent;
A step B of calcining the product obtained in the step A;
The method for producing a positive electrode active material comprises the steps of: The method according to claim 1 , wherein the temperature of the heat treatment in the step A is 100 to 400° C. The method according to claim 1 or 2 , wherein the firing temperature in the step B is 200 to 1000 ° C.

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