Anode active material for sodium-ion batteries and method for manufacturing the same, anode material and sodium-ion battery

Abstract

To provide a negative electrode active material capable of constructing a sodium ion battery that has a large capacity even after repeated use and has low charge transfer resistance.SOLUTION: A negative electrode active material for a sodium ion battery according to the present invention contains a sulfide of Co and a sulfide of a transition metal M, and the transition metal M is one selected from the group consisting of Mo, V, and W. A method for manufacturing a negative electrode active material for a sodium ion battery according to the present invention includes a step 1 of heating a raw material liquid containing a cobalt source and a transition metal M source to obtain a precursor, and a step 2 of heating the precursor in the presence of a sulfur source to generate sulfide.SELECTED DRAWING: None

Description

[Technical Field] 【0001】 This invention relates to a negative electrode active material for sodium-ion batteries, a method for producing the same, a negative electrode material, and a sodium-ion battery. [Background technology] 【0002】 While lithium-ion batteries (LIBs) are currently the mainstream of storage batteries, sodium-ion batteries (SIBs) have recently been gaining attention as one of the most promising future storage batteries. This is because SIBs have the greatest potential as a substitute for LIBs due to their abundant sodium resources, wide distribution, energy storage mechanism similar to LIBs, and low cost. 【0003】 In sodium-ion batteries, layered transition metal dichalcogenides (MS2, M=Mo,W,V,Ti or Sn, etc.) are attracting attention as novel battery materials. Such layered transition metal dichalcogenides possess physical properties such as large SMS interlayers and weak van der Waals forces, and it is believed that by repeatedly inserting and extracting Na ions, they can provide an effective ion diffusion pathway, thereby further improving the ion diffusion rate. 【0004】 From this perspective, for example, it has been proposed to apply cobalt sulfide (CoS, CoS2, Co3S4, Co9S8, etc.) as an active material for sodium-ion batteries (e.g., Non-Patent Document 1), and research is underway to improve it through morphological control, nanocrystallization, elemental substitution, hybridization, and defect introduction. [Prior art documents] [Non-patent literature] 【0005】 [Non-Patent Document 1] Acs Sustain. Chem. Eng. 2019, 7, 6122-6130. [Overview of the Initiative] [Problems that the invention aims to solve] 【0006】 However, even when cobalt sulfide is used as the negative electrode active material for sodium-ion batteries, the capacity after repeated use of the battery is insufficient, and the charge transfer resistance is also high. Therefore, there is a need for a negative electrode active material for sodium-ion batteries that can further improve the battery characteristics. 【0007】 The present invention has been made in view of the above, and aims to provide a negative electrode active material that can be used to construct a sodium-ion battery that has a large capacity even after repeated use and also has low charge transfer resistance. [Means for solving the problem] 【0008】 The inventors of this invention conducted extensive research to achieve the above objective and discovered that this objective can be achieved by using a sulfide having a Co sulfide phase and a specific transition metal M sulfide phase, thus completing the present invention. 【0009】 In other words, the present invention encompasses, for example, the subject matter described in the following sections. Item 1 It contains a sulfide of Co and a sulfide of a transition metal M. The transition metal M is one selected from the group consisting of Mo, V, and W, in the negative electrode active material for a sodium-ion battery. Section 2 The anode active material for a sodium-ion battery according to item 1, wherein the sulfide of Co is Co9S8, and the sulfide of the transition metal M is MS2 (where M is synonymous with the transition metal M). Section 3 A negative electrode active material for a sodium-ion battery according to item 1 or 2, which is particulate having a microflower structure. Section 4 A negative electrode material comprising the negative electrode active material for sodium-ion batteries described in item 1 or 2. Section 5 A sodium-ion battery comprising the negative electrode material described in item 4. Section 6 The method for manufacturing a negative electrode active material for a sodium ion battery according to Item 1 or 2, Step 1 of heating a raw material liquid containing a cobalt source and a transition metal M source to obtain a precursor, Step 2 of heat-treating the precursor in the presence of a sulfur source to generate a sulfide The method for manufacturing a negative electrode active material comprising: Item 7 The method for manufacturing a negative electrode active material according to Item 6, wherein the sulfide is in the form of particles having a microflower structure. 【Advantages of the Invention】 【0010】 The negative electrode active material for a sodium ion battery of the present invention can construct a sodium ion battery that has a large capacity even after repeated use and has a low charge transfer resistance. 【Brief Description of the Drawings】 【0011】 [Figure 1] SEM images of the negative electrode active materials obtained in Example 1 (Figs. 1(a) and (b)), Example 2 (Figs. 1(c) and (d)), and Example 3 (Figs. 1(e) and (f)). [Figure 2] SEM images of the negative electrode active materials obtained in Example 2 (Figs. 2(a) and (b)) and Comparative Example 1 (Figs. 2(c) and (d)). [Figure 3] Results of X-ray diffraction measurement (XRD) of the negative electrode active materials obtained in the examples and comparative examples. [Figure 4] Results of the constant current charge-discharge test of the batteries assembled in Preparation Example 1 (negative electrode active material of Example 2) and Preparation Example 2 (negative electrode active material of Comparative Example 1) are shown. [Figure 5] Evaluation results of the cycle characteristics with changes in each current density. [Figure 6] Results of the Nyquist plots of the batteries assembled in Preparation Example 1 (negative electrode active material of Example 2) and Preparation Example 2 (negative electrode active material of Comparative Example 1) are shown. 【Modes for Carrying Out the Invention】 【0012】 Embodiments of the present invention will be described in detail below. In this specification, the expressions "containing" and "including" include the concepts of "containing," "including," "substantially consisting of," and "consisting only of." 【0013】 1.Negative electrode active material The anode active material for sodium-ion batteries of the present invention (hereinafter simply referred to as "anode active material") comprises a sulfide of Co and a sulfide of a transition metal M, wherein the transition metal M is selected from the group consisting of Mo, V, and W. 【0014】 The negative electrode active material of the present invention can be used as an active material for forming a negative electrode material for a sodium-ion battery. By using the negative electrode active material of the present invention, a sodium-ion battery can be constructed that has a large capacity even after repeated use and also has low charge transfer resistance. In other words, by applying the negative electrode active material of the present invention to a sodium-ion battery, the cycle capacity characteristics can be improved. Therefore, the negative electrode active material of the present invention is suitable as an active material for a negative electrode material for a sodium-ion battery. 【0015】 In the anode active material of the present invention, the forms in which the Co sulfide and the transition metal M sulfide exist are not particularly limited. For example, the anode active material of the present invention may be a mixture of Co sulfide and the transition metal M sulfide. Alternatively, the anode active material of the present invention may be a composite sulfide of Co and the transition metal M, in which case it may have a structure in which, for example, the metal in the sulfide framework of one metal is substituted with the other metal. That is, the anode active material of the present invention may include a Co sulfide phase and a transition metal M sulfide phase. 【0016】 The presence of Co sulfide and transition metal M sulfide in the negative electrode active material of the present invention can be determined, for example, by XRD (X-ray diffraction spectrum measurement). Specifically, the presence of Co sulfide and transition metal M sulfide in the negative electrode active material can be confirmed by checking the presence or absence of signals based on the Co sulfide phase and signals based on the transition metal M sulfide phase in the XRD spectrum of the negative electrode active material of the present invention. 【0017】 In the negative electrode active material of the present invention, the transition metal M is more preferably Mo. In this case, the negative electrode active material can provide excellent cycle capacity characteristics to the sodium-ion battery. Therefore, a preferred embodiment of the present invention is a negative electrode active material comprising a sulfide of Co and a sulfide of Mo. 【0018】 Examples of Co sulfides include CoS, CoS2, Co3S4, or Co9S8, among which Co9S8 is preferred because it can provide superior cycle capacity characteristics in sodium-ion batteries. 【0019】 Examples of transition metal M sulfides include MS, MS2, M3S4, or M9S8. Among these, MS2 is preferred, and MoS2 is more preferred, as it can provide superior cycle capacity characteristics in sodium-ion batteries. 【0020】 Therefore, in the negative electrode active material of the present invention, it is more preferable that the sulfide of Co is Co9S8 and the sulfide of the transition metal M is MS2 (where M is the same as the transition metal M), and it is even more preferable that the sulfide of Co and the sulfide of the transition metal M are Co9S8 and MoS2, respectively. 【0021】 In the present invention, it is preferable that the negative electrode active material exhibits both a Co9S8 phase and a MoS2 phase in its XRD spectrum. 【0022】 In the negative electrode active material of the present invention, the content ratios of Co, transition metal M, and sulfur are not particularly limited. For example, the content ratio of Co is preferably 20 to 50% by mass, and more preferably 30 to 40% by mass, relative to the total mass of Co, transition metal M, and sulfur. Furthermore, the content ratio of transition metal M is preferably 3 to 20% by mass, and more preferably 5 to 15% by mass, relative to the total mass of Co, transition metal M, and sulfur. 【0023】 In the negative electrode active material of the present invention, the ratio of Co to transition metal M is not particularly limited. For example, the atomic ratio of Co to transition metal M is preferably 1:0.1 to 1:10, more preferably 1:0.2 to 1:5, more preferably 1:0.3 to 1:1, and even more preferably 1:0.4 to 1:0.8. 【0024】 The negative electrode active material of the present invention may contain elements other than Co, transition metal M, and sulfur, and may also contain other components. The negative electrode active material of the present invention preferably contains 80% by mass or more of Co sulfide and transition metal M sulfide, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 99% by mass or more. The negative electrode active material of the present invention may consist only of Co sulfide and transition metal M sulfide. 【0025】 The form of the negative electrode active material of the present invention is not particularly limited and can take various forms such as powder, lump, granule, and fibrous form. When the negative electrode active material of the present invention is in powder form, for example, it can be particulate, and specifically, it may have various shapes such as porous particles, hollow particles, amorphous particles, and spherical particles. Among these, the negative electrode active material of the present invention is preferably porous particles, and more preferably particulate with a microflower structure. In this case, a sodium-ion battery can be provided with even better cycle capacity characteristics. Particles having a microflower structure can mean, for example, petal-shaped like a hydrangea, with a diameter of 500 μm or less. The microflower structure may also be tremella-like. 【0026】 When the negative electrode active material of the present invention is in particulate form, its average particle diameter is not particularly limited and can be, for example, 100 nm to 500 μm, preferably 500 nm to 300 μm, more preferably 1 to 80 μm, and even more preferably 2 to 50 μm. The average particle diameter referred to here is the value obtained by arithmetic mean of 50 particles randomly selected by direct observation of the negative electrode active material with a scanning electron microscope and measuring their equivalent circle diameters. 【0027】 Conventional layered transition metal dichalcogenides (MS2), such as CoS2, exhibit large volume fluctuations and low electronic conductivity, resulting in insufficient rate performance and cycle stability. In contrast, the anode active material of the present invention, by incorporating Co sulfide and transition metal M sulfide as essential components, can provide excellent cycle capacity characteristics to sodium-ion batteries. Furthermore, its low charge transfer resistance is expected to improve reaction rates. Therefore, the anode active material of the present invention is suitable as an active material for anode materials used in sodium-ion batteries. 【0028】 2. Method for producing the negative electrode active material The method for producing the negative electrode active material of the present invention is not particularly limited. For example, the negative electrode active material of the present invention can be produced by a production method comprising the following steps 1 and 2. Step 1: A step of heating a raw material liquid containing a cobalt source and a transition metal M source to obtain a precursor. Step 2: A step of heating the precursor in the presence of a sulfur source to produce a sulfide. 【0029】 (Process 1) Step 1 is a step of heating a raw material liquid containing a cobalt source and a transition metal M source, and by this step, a precursor of the negative electrode active material of the present invention is obtained. 【0030】 The cobalt source may be elemental Co, a compound of Co, or a mixture thereof, and is preferably a compound of Co. 【0031】 The types of compounds containing Co are not particularly limited; for example, inorganic compounds, chlorides, and organic compounds of Co can be cited. Examples of inorganic compounds of Co include nitrates, sulfates, chlorides, oxides, chlorates, perchlorates, chloride complexes, carbonates, bicarbonates, phosphates, and hydrogen phosphates of Co, as well as cyanide compounds, cyanide compound salts, and compounds containing oxoanions (cobaltates). Examples of organic compounds of Co include acetates, oxalates, formates, and succinates. 【0032】 In particular, the cobalt source is preferably an inorganic compound of Co, and more preferably a cyanide compound salt of Co (for example, potassium hexacyanocobalt(III)ate; K3[Co(CN)6]). 【0033】 The transition metal M source may be the transition metal element M in itself, or a compound containing the transition metal element M, but it is preferable that it be a compound containing the transition metal element M. 【0034】 The type of compound containing the transition metal element M is not particularly limited, and examples include various inorganic compounds containing the transition metal element M. Examples of inorganic compounds containing the transition metal element M include oxides of the transition metal element M, compounds containing the oxoanion of the transition metal element M (metal salts), as well as nitrates, sulfates, chlorides, chlorates, perchlorates, carbonates, bicarbonates, phosphates, and hydrogen phosphates of the transition metal element M. Among these, it is preferable that the inorganic compound containing the transition metal element M is a compound containing the oxoanion of the transition metal element M. 【0035】 Examples of compounds containing oxoanions of specific transition metal elements M include vanadates, molybdates, and tungstates. The type of salt is not particularly limited and includes, for example, ammonium salts and alkali metal salts. Examples of vanadates include sodium orthovanadate(V) and sodium metavanadate (NaVO3). Examples of molybdates include sodium molybdate (Na2MoO4). An example of a tungstate is sodium tungstate (Na2WO4). 【0036】 The compound containing the transition metal element M may be various organic compounds of the transition metal element M, for example, acetate, oxalate, formate, and succinate salts of the transition metal element M. 【0037】 The raw material liquid used in step 1 includes the cobalt source and the transition metal M source, as well as a solvent. Water can be used as the solvent, 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 1 may also be a mixture of water and an organic solvent. Water is preferred as the solvent used in step 1. 【0038】 The solvent may contain various additives. These additives may include aqueous ammonia, as well as dispersion stabilizers and surfactants such as polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB), sodium lauryl sulfonate (SDS), and ethylenediaminetetraacetic acid (EDTA). The inclusion of additives in the solvent makes it easier to obtain a positive electrode active material having the aforementioned layer-by-layer structure. When the solvent contains additives, the proportion 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. 【0039】 In the raw material solution used in step 1, the proportions of the cobalt source, the transition metal M source, and the solvent are not particularly limited. For example, in the raw material solution used in step 1, the concentrations of the cobalt source and the transition metal M source can be 0.1 to 100 mM, preferably 0.2 to 50 mM, and more preferably 0.5 to 10 mM. 【0040】 In step 1, the raw material liquid is heat-treated. One method of heat treatment is to place the raw materials in a container, seal the container, and heat the contents of the container. If the solvent in the raw materials is water, this is a so-called hydrothermal synthesis method. 【0041】 The temperature inside the container during the heat treatment in step 1 is not particularly limited and can be, for example, 100 to 400°C, with 150 to 250°C being preferable. The heating time is also not particularly limited and can be appropriately determined according to the heating temperature, for example, 6 to 40 hours. The pressure inside the container during the heat treatment can also be set appropriately. 【0042】 In step 1, 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 methods such as centrifugation, washed, and dried to obtain the product as a solid. 【0043】 In step 1, the precursor obtained by heat treatment is preferably freeze-dried. The freeze-drying method is not particularly limited, and for example, known freeze-drying conditions can be widely applied. 【0044】 (Process 2) Step 2 is a step for generating sulfides by heating the precursor obtained in Step 1 in the presence of a sulfur source. 【0045】 The sulfur source used in step 2 may be elemental sulfur or a sulfur-containing compound. The sulfur source is preferably elemental sulfur, and more preferably sulfur powder. 【0046】 Examples of sulfur-containing compounds include a wide range of known sulfur compounds, such as thioacetamide (CH3CSNH2), thiourea (SC(NH2)2), cysteine ​​(C3H7NO2S), sodium thiosulfate (Na2S2O3), ammonium sulfide ((NH4)2S), and sodium sulfide (Na2S). In addition, in sulfur-containing compounds, some of the sulfur element may be replaced with Se and / or Te. 【0047】 In step 2, one sulfur source can be used alone, or two or more can be used in combination. 【0048】 In step 2, the method for heat-treating the precursor in the presence of a sulfur source is not particularly limited, and for example, a wide range of known sulfidation methods can be employed. Among these, a solid sulfidation method is preferred. Specifically, a method of heat-treating the space in which a solid sulfur source and a solid precursor are present is preferred. In this method, for example, a container containing a solid sulfur source and a container containing the solid precursor obtained in step 1 are placed in a sealed container, and a sulfide can be produced by heat-treating this sealed container. 【0049】 The temperature of the heat treatment in step 2 can be, for example, 150 to 700°C, preferably 200 to 600°C, and more preferably 300 to 550°C. The heat treatment time can be set appropriately according to the temperature, etc., for example, 1 to 5 hours. 【0050】 The heat treatment in step 2 can be carried out, for example, in an inert gas atmosphere, in which case the inert gas may contain hydrogen gas. If the inert gas contains hydrogen gas, the hydrogen gas content can be 5 to 30 volume percent of the total amount of inert gas. 【0051】 The proportions of the precursor and sulfur source used in step 2 are not particularly limited, and an excess amount of sulfur source can be used relative to the precursor. 【0052】 Through the above step 2, the precursor is subjected to sulfidation treatment, and the desired sulfide can be obtained. Such a sulfide contains a sulfide of Co and a sulfide of the transition metal M, and can serve as the negative electrode active material of the present invention as described above. Therefore, the sulfide obtained in step 2 may be in the form of particulate matter having the aforementioned microflower structure. 【0053】 3. Anode material The negative electrode material of the present invention may contain other components as long as it contains the above-mentioned negative electrode active material. For example, known components used in the negative electrode material of sodium-ion batteries can be cited. For example, the negative electrode material of the present invention may contain a conductive additive and a binder in addition to the above-mentioned negative electrode active material. 【0054】 Conductive additives can broadly include known conductive additives used, for example, to form electrode materials for various types of batteries. Examples of conductive additives include various carbon materials, such as hard carbon, soft carbon, graphene, reduced graphene oxide, natural graphite, artificial graphite, conductive carbon black, and carbon fibers. Examples of carbon fibers include carbon nanofibers and carbon nanotubes. Other conductive additives that can be used include metal powders such as copper and nickel, metal fibers, and conductive ceramic materials. 【0055】 Binders can be broadly categorized into known binders used to form electrode materials for various types of batteries. 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, and the like. 【0056】 In the negative electrode material, the content ratio of the negative electrode active material is not particularly limited. For example, it is preferable that the negative electrode active material is contained in an amount of 50 to 95% by mass, and more preferably 60 to 90% by mass, relative to the total mass of the negative electrode active material, conductive additive, and binder contained in the negative electrode material. 【0057】 In the negative electrode material, the proportion of the conductive additive is not particularly limited. For example, it is preferable that the conductive additive is present in an amount of 3 to 30% by mass, and more preferably 5 to 20% by mass, relative to the total mass of the negative electrode active material, conductive additive, and binder contained in the negative electrode material. 【0058】 In the negative electrode material, the binder content is not particularly limited. For example, it is preferable that the binder be present in an amount of 3 to 30% by mass, and more preferably 5 to 20% by mass, relative to the total mass of the negative electrode active material, conductive additive, and binder contained in the negative electrode material. 【0059】 The negative electrode material may consist only of a negative electrode active material, a conductive additive, and a binder, or it may contain other components. 【0060】 The method for preparing the negative electrode material is not particularly limited, and for example, known methods for preparing negative electrode materials can be widely employed. For example, the negative electrode material can be prepared by mixing a negative electrode active material, a conductive additive, and a binder in predetermined proportions using an appropriate method. When preparing the negative electrode material, a solvent can also be used to disperse the negative electrode active material, conductive additive, and binder. Examples of solvents include water and various organic solvents, such as lower alcohol compounds with 1 to 3 carbon atoms, and NMP (N-methyl-2-pyrrolidone). When the negative electrode material contains a solvent, it may be in the form of a slurry or paste. 【0061】 4. Sodium-ion battery As long as the sodium-ion battery of the present invention includes the negative electrode material, the other configurations are not particularly limited, and for example, it can have a configuration similar to that of a known sodium-ion battery. The sodium battery of the present invention is more preferably a sodium-ion secondary battery. 【0062】 A sodium-ion battery may comprise, for example, a positive electrode, a negative electrode, an electrolyte, and a separator. The size and shape of the battery can be appropriately determined according to its application. 【0063】 The positive electrode can have a structure composed of, for example, a metal foil and a positive electrode material. Examples of metals for forming the metal foil include aluminum, titanium, platinum, molybdenum, stainless steel, and copper. The positive electrode material can be a wide range of known positive electrode materials; for example, materials constituting the positive electrode material include sodium metal, NaFePO4, Na3V2(PO4)3, and Na x Examples include MO4 (M=Co, Mn, V, Fe). The positive electrode can be manufactured by known methods, such as coating a positive electrode material onto a metal foil. 【0064】 The negative electrode may have a structure in which the negative electrode active material of the present invention is supported on a metal foil, for example. Examples of metal foils include aluminum, titanium, platinum, molybdenum, stainless steel, and copper. The negative electrode can be manufactured by known methods. 【0065】 In sodium-ion batteries, the type of electrolyte is not particularly limited; for example, known electrolytes can be used. The electrolyte may be either a solid electrolyte or a liquid electrolyte. 【0066】 Liquid electrolytes include solutions in which an electrolyte is dissolved in a solvent. Examples of electrolytes include various sodium salts, such as NaPF6, NaClO4, NaCF3SO3, NaFSI, and NaTFSI. Examples of solvents include water, diglyme, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, propyl acetate, fluoroethylene carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. 【0067】 Examples of solid electrolytes include inorganic materials such as sulfides and oxides, and polymer materials such as PEO (polyethylene oxide). 【0068】 As the separator, known separators used in secondary batteries can be used, such as polyolefin resins such as polyethylene and polypropylene; polyimide; polyvinyl alcohol; fluororesins such as terminally aminated polyethylene oxide polytetrafluoroethylene; acrylic resin; nylon; aromatic aramid; inorganic glass; ceramics, etc. The separator can be in the form of a porous membrane, nonwoven fabric, woven fabric, etc. Other separators include various polymer membranes and inorganic electrolytes. Examples of inorganic electrolytes include LiLaTiO3 and Li7La3Zr2O 12 (LLZO), Na3Zr2Si2PO 12 kaNa 11 Sn2PS 12 Examples include Na3PSe4. [Examples] 【0069】 The present invention will be described more specifically below with reference to examples, but the present invention is not limited to the embodiments of these examples. 【0070】 (Example 1) As a cobalt source, 30 mL of a 1 mM K3[Co(CN)6] aqueous solution A was prepared, and as a transition metal M source, 30 mL of a 0.6 mM Na2MoO4 aqueous solution B was prepared. Aqueous solution B was added to aqueous solution A, and the mixture was stirred for 1 hour to obtain the raw material solution. This raw material solution was transferred to a 100 mL autoclave lined with Teflon®, the autoclave was sealed, and the mixture was heated at 200°C for 20 hours to perform hydrothermal synthesis and obtain the precursor (Step 1). This precursor was collected by centrifugation and then freeze-dried for 24 hours to obtain the precursor powder. Next, 1 g of sulfur powder and 0.1 g of the precursor powder were placed upstream and downstream of the reaction tube, respectively, and the mixture was heated at 500°C for 3 hours while argon gas (containing 20% ​​by volume of hydrogen gas) was flowed from upstream to downstream (solid sulfidation). This obtained the desired sulfide as the negative electrode active material (Step 2). The obtained negative electrode active material was denoted as "CM53". 【0071】 (Example 2) The negative electrode active material was obtained in the same manner as in Example 1, except that the concentration of aqueous solution B of Na2MoO4 was changed to 1.0 mM. The obtained negative electrode active material was denoted as "CM55". 【0072】 (Example 3) The negative electrode active material was obtained in the same manner as in Example 1, except that the concentration of Na2MoO4 aqueous solution B was changed to 1.4 mM. The obtained negative electrode active material was denoted as "CM57". 【0073】 (Comparative Example 1) The negative electrode active material was obtained in the same manner as in Example 1, except that the transition metal M source (Na2MoO4 aqueous solution B) was not used. The obtained negative electrode active material was denoted as "CM50". 【0074】 (Example 1) A battery was fabricated using the negative electrode active material obtained in Example 2. Specifically, a slurry for the negative electrode material was prepared consisting of the negative electrode active material, superP (conductive carbon black) as a conductive additive, and polyvinylidene fluoride (PVDF) dissolved in methylpyrrolidinone as a binder. In this slurry, the ratio of negative electrode active material:superP:PVDF was 7.5:1.5:1 (mass ratio). A negative electrode with a diameter of 12 mm was fabricated by coating copper foil with the slurry and drying it in a vacuum at 80°C for 12 hours. The battery was assembled using this negative electrode, a positive electrode (sodium metal with aluminum foil), and a separator ("Whatman GF / C glass fiber filter paper" provided by Cytiva) impregnated with a liquid electrolyte, using a known method. The electrolyte was a 1 M sodium trifluoromethanesulfonate solution, and the solvent for this solution was diglyme. 【0075】 (Example 2) The battery was assembled in the same manner as in Example 1, except that the negative electrode active materials obtained in Example 2 were replaced with the negative electrode active materials obtained in Comparative Example 1. 【0076】 (Evaluation results) Figure 1 shows SEM images of the negative electrode active materials obtained in Example 1 (Figures 1(a) and (b)), Example 2 (Figures 1(c) and (d)), and Example 3 (Figures 1(e) and (f)). 【0077】 Figure 2 also shows SEM images of the negative electrode active materials obtained in Example 2 (Figures 2(a) and (b)) and Comparative Example 1 (Figures 2(c) and (d)). 【0078】 The negative electrode active materials obtained in Examples 1-3 were all found to be porous particulates, and in particular, they were found to have a microflower structure similar to tremella. The negative electrode active material obtained in Comparison 1 had a sea urchin-like structure assembled from independent nanobelts. 【0079】 Although not shown in the illustration, as a result of examining the precursor for obtaining the negative electrode active material of the embodiment by EDS elemental mapping, it was shown that Co, Mo, and N were uniformly distributed in the precursor, and the metal atom ratio (Co:Mo) of Co and Mo was close to 2:1 in all samples (CM53 has Co at 46.7 At%, Ni at 29.2 At%, and N at 24.2 At%, CM55 has Co at 46.5 At%, Ni at 30.5 At%, and N at 23 At%, and CM57 has Co at 52.4 At%, Ni at 24.9 At%, and N at 22.7 At%). Therefore, it is presumed that the negative electrode active materials of Examples 1 to 3 can all take similar forms. 【0080】 Figure 3 shows the results of X-ray diffraction measurement (XRD) of the negative electrode active materials obtained in the examples and comparative examples. For the X-ray diffraction measurement, “SmartLab” manufactured by Rigaku 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°. 【0081】 From the XRD patterns in Figure 3, the presence of both the Co9S8 phase and the MoS2 phase was confirmed in the negative electrode active materials obtained in Examples 1 to 3. In contrast, the negative electrode active material obtained in Comparative Example 1 was the Co9S8 phase. 【0082】 From the above XRD patterns, it was shown that the negative electrode active materials obtained in Examples 1 to 3 contain Co9S8 and MoS2. 【0083】 Figure 4 shows the results of the constant current charge-discharge test of the batteries assembled in Preparation Example 1 (the negative electrode active material of Example 2) and Preparation Example 2 (the negative electrode active material of Comparative Example 1). For this measurement, it was measured using the LAND battery test system “CT2001A” (Wuhan LAND electronics Co., Ltd. China). Here, the measurement temperature was 30°C, and the applied voltage was 0.3 to 3 V (Figure 4(a) is 1Ag -1 , Figure 4(b) is 2Ag -1 , Figure 4(3) is 5Ag -1 ). In Figure 4, the first axis of the Y-axis indicates the capacity (mAhg -1 ), and the second axis of the Y-axis indicates the Coulomb efficiency (%). 【0084】 From Figure 4(a), the battery using the negative electrode active material obtained in Example 2 (Fabrication Example 1) has 1Ag -1 At that time, 493.7mAhg after 300 cycles -1 It was found to have a capacity of 114.3%. On the other hand, the battery using the negative electrode active material obtained in Comparative Example 1 (Fabrication Example 2) had a capacity of 402.2 mAhg under the same conditions. -1 With this capacity, the capacity retention rate remained at 80.4%. Also, the battery in example 1 was 2Ag -1 and 5Ag -1 It demonstrated excellent cycle performance even at such high current densities. 【0085】 Therefore, it was found that the negative electrode active material obtained in Example 2 can be used to construct a sodium-ion battery that has a large capacity even after repeated use and also has low charge transfer resistance. 【0086】 Figure 5 shows the evaluation results of the cycle characteristics of the battery assembled in Fabrication Example 1 as the current density changes. The battery in Fabrication Example 2 has current densities of 0.1 to 0.2, 0.5, 1.0, 2.0 and 5.0 Ag. -1 When increased to these values, the values ​​were 439.2, 429.1, 417.3, 404.8, 387.4, and 338.2 mAhg, respectively. -1 This allowed us to provide an average discharge capacity of 0.1Ag. Although this is comparable to the battery in Fabrication Example 2, as shown in Figure 5(b), the current density was increased to 0.1Ag. -1 Returning to the original state and continuing the charge-discharge process for 160 cycles, the battery capacity of Example 2 is 539.5 mAhg -1 It recovered to 0.1Ag in the first 5 cycles. -1 The average capacity was significantly higher than in the previous example. This result also clearly shows that the battery using the negative electrode active material obtained in Example 2 has excellent cycle characteristics. 【0087】 Figure 6 shows the Nyquist plot results for batteries assembled using Fabrication Example 1 (negative electrode active material of Example 2) and Fabrication Example 2 (negative electrode active material of Comparative Example 1). 【0088】 Figure 6 shows that the charge transfer resistance (Rct) of the battery in example 1 was 4.02 Ω, while that of the battery in example 2 was 10.59 Ω. The Rs values ​​were 14.61 Ω for example 1 and 17.45 Ω for example 2. A lower Rct value indicates a faster reaction rate; therefore, it is suggested that the battery in example 1 has high conductivity and a rational heterostructure.

Claims

[Claim 1] A compound comprising Co₇S₄ and a sulfide of a transition metal M, The transition metal M is one selected from the group consisting of Mo, V, and W. The sulfide of the transition metal M is MS2 (where M is synonymous with the transition metal M), and this is a negative electrode active material for a sodium-ion battery. [Claim 2] The negative electrode active material for a sodium-ion battery according to claim 1, which is particulate having a microflower structure. [Claim 3] A negative electrode material comprising the negative electrode active material for a sodium-ion battery according to claim 1 or 2. [Claim 4] A sodium-ion battery comprising the negative electrode material described in claim 3. [Claim 5] A method for producing a negative electrode active material for a sodium ion battery according to claim 1 or 2, Step 1 involves heating a raw material liquid containing a cobalt source and a transition metal M source to obtain a precursor, Step 2 involves heating the precursor in the presence of a sulfur source to produce a sulfide. A method for producing a negative electrode active material, comprising the same components. [Claim 6] The method for producing a negative electrode active material according to claim 5, wherein the sulfide is in the form of particulate matter having a microflower structure.

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