Positive electrode material, preparation method thereof and secondary battery

Abstract

The invention relates to a positive electrode material, a preparation method thereof and a secondary battery. The positive electrode material comprises an inner core, a coating layer arranged on the surface of the inner core, and tin disulfide nanosheets at least covering part of the coating layer, the inner core comprises an iron disulfide inner core, the coating layer comprises a carbon coating layer, and the carbon coating layer is combined with the tin disulfide nanosheets through chemical bonds. Therefore, the deformation of an inner core in a battery circulation process can be effectively inhibited, the conductivity of the positive electrode material is improved, meanwhile, the specific surface area of the positive electrode material is remarkably increased, the permeation of an electrolyte is accelerated, more active sites are provided, the electron transmission distance is shortened, and the diffusion of active ions is promoted; and the capacity and the cycle performance of the battery can be well improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a cathode material, a preparation method thereof, and a secondary battery. Background Art

[0002] Lithium-ion batteries are currently widely used electrochemical energy storage devices. However, due to the relatively active nature of lithium, lithium dendrites are likely to form on the surface of the battery negative electrode, leading to safety problems. Coupled with the limited lithium resources, the further development of lithium-ion batteries is restricted. Sodium and potassium are abundant in the earth's crust and have relatively low potentials. Therefore, sodium-ion and potassium-ion batteries are also being developed one after another. However, due to the problems of large size and low reversibility of sodium ions and potassium ions, the selection of electrode materials is greatly restricted, affecting the application of the batteries. In other ion battery systems, such as calcium ions, they have a relatively large ionic radius, which will hinder their smooth insertion into the host lattice. Although zinc ions have a small radius and a large capacity, they are relatively scarce in the earth's crust, and it is difficult to achieve good applications for both.

[0003] Magnesium resources are relatively abundant, and magnesium metal has a high capacity (about 3833 mAh / cm 3 ). Rechargeable magnesium-ion batteries have received much attention in recent years. Magnesium metal negative electrodes have the advantage of uniform deposition, which can effectively avoid the growth of dendrites on the negative electrode surface. The energy storage of magnesium-ion batteries is achieved through the charging and discharging of electrons and ions. During the charging and discharging process, electrochemical reactions occur on the electrodes. Therefore, the performance of the electrode materials plays a decisive role in the performance of the battery.

[0004] However, due to the small volume and strong polarization of magnesium ions, most cathode materials that can reversibly store lithium ions are not suitable for reversibly storing magnesium ions. A suitable cathode material for magnesium-ion batteries can ensure that each transition metal center reversibly inserts or extracts a magnesium ion, providing two electrons, thereby obtaining a capacity about twice that of the cathode of lithium-ion batteries. However, the strong electrostatic interaction between magnesium ions and anions in the lattice of the cathode material usually greatly reduces the insertion and diffusion kinetics of magnesium ions, resulting in a decrease in the reversible capacity and cycling performance of magnesium-ion batteries. Therefore, developing cathode materials with better performance is an urgent problem to be solved at present. Summary of the Invention

[0005] In view of this, embodiments of the present application provide a cathode material, a preparation method thereof, and a secondary battery to solve at least one problem in the background art.

[0006] In a first aspect, embodiments of the present application provide a cathode material, including a core, a coating layer disposed on the surface of the core, and tin disulfide nanosheets covering at least a part of the coating layer;

[0007] The core includes an iron disulfide core, and the coating layer includes a carbon coating layer. The carbon coating layer is chemically bonded to the tin disulfide nanosheets.

[0008] Combined with the first aspect of the present application, in an alternative embodiment, the positive electrode material satisfies at least one of the following characteristics:

[0009] (1) The tin disulfide nanosheets include metal doping elements; optionally, the metal doping elements include at least one of tungsten, vanadium, and niobium;

[0010] (2) The carbon coating layer includes a nitrogen-doped carbon coating layer;

[0011] (3) The particle size of the core is 0.5 μm to 3 μm;

[0012] (4) The thickness of the coating layer is 3 nm to 20 nm;

[0013] (5) The mass ratio of the tin disulfide nanosheets to the core is 1:2 to 1:10;

[0014] (6) The thickness of the tin disulfide nanosheets is 10 nm to 50 nm.

[0015] In a second aspect, an embodiment of the present application provides a method for preparing a positive electrode material, the method comprising the following steps:

[0016] S1: Add a tin source, a first sulfur source, and iron disulfide with a coating layer to a mixed solution of water and alcohols, perform a first reaction to form tin disulfide nanosheets on the coating layer, and after washing and drying, obtain the positive electrode material; the coating layer includes a carbon coating layer, and the carbon coating layer is chemically bonded to the tin disulfide nanosheets.

[0017] Combined with the second aspect of the present application, in an alternative embodiment, in step S1, a metal salt is further added to the mixed solution, and the formed tin disulfide nanosheets include metal doping elements; optionally, the metal salt includes at least one of ammonium metatungstate, sodium metavanadate, and niobium pentachloride.

[0018] Combined with the second aspect of the present application, in an alternative embodiment, step S1 satisfies at least one of the following characteristics:

[0019] (1) The tin source includes tin chloride and / or tin oxalate;

[0020] (2) The first sulfur source includes at least one of sulfur powder, thioacetamide, and thiourea;

[0021] (3) The volume ratio of water to alcohols in the mixed solution is 1:15 to 15:1;

[0022] (4) The temperature of the first reaction is 160°C to 180°C;

[0023] (5) The time of the first reaction is 4 h to 12 h;

[0024] (6) The temperature of the drying is 60°C to 80°C.

[0025] Combined with the second aspect of the present application, in an alternative embodiment, the preparation method of the iron disulfide with a coating layer includes:

[0026] Dissolve dopamine hydrochloride in an alkaline solution, mix evenly to obtain a mixed solution;

[0027] Add iron disulfide to the mixed solution, stir to form a polydopamine coating layer on the surface of the iron disulfide, and dry to obtain a reaction product;

[0028] Under the atmosphere of a protective gas, perform carbonization treatment on the reaction product to obtain the iron disulfide with a coating layer.

[0029] Combined with the second aspect of the present application, in an alternative embodiment, the method satisfies at least one of the following features:

[0030] (1) The mass ratio of the dopamine hydrochloride to the iron disulfide is 1:1 to 2:1;

[0031] (2) The alkaline solution includes ammonia water and / or sodium hydroxide solution;

[0032] (3) The stirring time is 24 h to 48 h;

[0033] (4) The protective gas includes at least one of nitrogen, argon, and helium;

[0034] (5) The temperature of the carbonization treatment is 350°C to 600°C;

[0035] (6) The time of the carbonization treatment is less than or equal to 10 h;

[0036] (7) The heating rate of the carbonization treatment is 0.5°C / min to 20°C / min;

[0037] (8) The cooling rate of the carbonization treatment is 0.5°C / min to 50°C / min.

[0038] Combined with the second aspect of the present application, in an alternative embodiment, the preparation method of the iron disulfide includes:

[0039] Add an iron source, a second sulfur source, and urea to a solvent, stir evenly and then perform a second reaction. After washing and drying the reaction product, obtain the iron disulfide.

[0040] Combined with the second aspect of the present application, in an alternative embodiment, the method satisfies at least one of the following features:

[0041] (1) The molar ratio of the iron source to the second sulfur source is 1:3 to 1:10;

[0042] (2) The molar ratio of the iron source to the urea is 1:5 to 1:10;

[0043] (3) The iron source includes at least one of iron sulfate, iron nitrate, and iron chloride;

[0044] (4) The second sulfur source includes at least one of sulfur powder, thiourea, and thioacetamide;

[0045] (5) The solvent includes at least one of deionized water, ethanol, ethylene glycol, and N,N-dimethylformamide;

[0046] (6) The temperature of the second reaction is 150°C to 200°C;

[0047] (7) The time of the second reaction is 12 h to 48 h.

[0048] In a third aspect, an embodiment of the present application provides a secondary battery, including a positive electrode sheet, and the positive electrode sheet includes the positive electrode material according to any one of the first aspect or the positive electrode material prepared by the preparation method of the positive electrode material according to any one of the second aspect.

[0049] Compared with the prior art, the embodiments of the present application have the following beneficial effects:

[0050] The positive electrode material, its preparation method, and the secondary battery provided by the embodiments of the present application. The positive electrode material includes a core, a coating layer provided on the surface of the core, and tin disulfide nanosheets covering at least a part of the coating layer; the core includes an iron disulfide core, and the coating layer includes a carbon coating layer. The carbon coating layer and the tin disulfide nanosheets are chemically bonded. In the embodiments of the present application, the coating layer provided on the surface of the core includes a carbon coating layer, and the carbon material has excellent electrical conductivity. Therefore, the coating layer can not only effectively inhibit the deformation of the core during the battery cycle, but also improve the electrical conductivity of the positive electrode material well. The sulfur vacancies on the surface of the tin disulfide nanosheets can form C-S bonds with carbon, so that the tin disulfide nanosheets can be tightly bonded to the carbon coating layer through chemical bonds. The tin disulfide nanosheets cover at least a part of the coating layer, so that the specific surface area of the positive electrode material is significantly increased, which can accelerate the penetration of the electrolyte, provide more active sites, shorten the electron transport distance, and promote the diffusion of active ions. Moreover, tin disulfide has a significantly higher specific capacity than iron disulfide. Therefore, it can improve the capacity and cycle performance of the battery well.

[0051] Additional aspects and advantages of the present application will be given in part in the following description, become apparent in part from the following description, or be learned by practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The drawings described herein are used to provide a further understanding of the present application, form a part of the present application, and the schematic embodiments and descriptions thereof are used to explain the present application, and do not constitute an improper limitation to the present application. In the drawings:

[0053] Figure 1 is a schematic flow chart of a method for preparing a cathode material provided by an embodiment of the present application;

[0054] Figure 2 is a schematic flow chart of a method for preparing iron disulfide with a coating layer provided by an embodiment of the present application;

[0055] Figure 3 is a scanning electron microscope image of the cathode material obtained in Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0056] In order to make the technical solutions and beneficial effects of the present invention more obvious and understandable, the following will be described in detail by combining the drawings and listing specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. The experimental methods without specific conditions noted in the following embodiments are generally in accordance with conventional experimental conditions. The reagents and raw materials used in the present invention are commercially available unless otherwise specified.

[0057] In the following description, a large number of specific details are given to provide a more thorough understanding of the present application. However, it is obvious to those skilled in the art that the present application can be implemented without one or more of these details. In other examples, in order to avoid confusion with the present application, some technical features well known to those skilled in the art are not described; that is, not all features of the actual embodiments are described here, and the well-known functions and steps are not described in detail.

[0058] The purpose of the terms used herein is only to describe specific embodiments and is not a limitation of the present application. When used herein, the singular forms "a", "an" and "the" are also intended to include the plural forms unless the context clearly indicates otherwise. The terms "comprising" and / or "including", when used in this specification, determine the presence of the described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups. When used herein, the term "and / or" includes any and all combinations of the related listed items.

[0059] To thoroughly understand this application, detailed steps and structures will be presented in the following description to illustrate the technical solutions of this application. The preferred embodiments of this application are described in detail below. However, in addition to these detailed descriptions, this application may have other implementation manners.

[0060] Unless otherwise defined, the technical and scientific terms used in this application have the same meanings as those of the technical and scientific terms in the technical field to which this application belongs.

[0061] For those not specified with specific technologies or conditions in the following embodiments, they are generally carried out according to the conventional technologies or conditions described in the literature in this field, or according to the conditions recommended in the product manuals and by the manufacturers. The numerical ranges in the following embodiments all include the end point values.

[0062] In the related technologies, various cathode materials that can be used in magnesium ion batteries have been studied, such as transition metal oxides, transition metal chalcogenides, transition metal halides, halogens, and polyanion-based compounds, etc. Considering the strong polarization effect of magnesium ions with relatively small ionic radii, chalcogen elements (such as S, Se) and chalcogen compounds have attracted the attention of researchers due to the relatively low electronegativity of chalcogen anions. Among various transition metal sulfides, iron disulfide (FeS2) and tin disulfide (SnS2) are used as an electrochemical energy storage material with high energy density, rich resources, and environmental friendliness in the cathode material. However, during the charge and discharge process of the battery, the volume change and poor conductivity of FeS2 and SnS2 will lead to a serious decline in the electrochemical performance of the battery.

[0063] Based on this, the embodiment of this application provides a cathode material, which includes a core, a coating layer disposed on the surface of the core, and tin disulfide nanosheets that at least cover part of the coating layer; the core includes an iron disulfide core, and the coating layer includes a carbon coating layer, and the carbon coating layer is chemically bonded to the tin disulfide nanosheets.

[0064] In the embodiment of this application, the coating layer disposed on the surface of the core includes a carbon coating layer. The carbon material has excellent conductivity. Therefore, the coating layer can not only effectively inhibit the deformation of the core during the battery cycle, but also better improve the conductivity of the cathode material. The sulfur vacancies on the surface of the tin disulfide nanosheets can form C-S bonds with carbon, so that the tin disulfide nanosheets can be tightly bonded to the carbon coating layer through chemical bonds. The tin disulfide nanosheets at least cover part of the coating layer, and an open framework structure can be formed, which significantly increases the specific surface area of the cathode material, can accelerate the penetration of the electrolyte, provide more active sites, shorten the electron transport distance, and promote the diffusion of active ions (such as magnesium ions, sodium ions, lithium ions, etc.). Moreover, tin disulfide has a significantly higher specific capacity than iron disulfide. Therefore, it can better improve the capacity and cycle performance of the battery.

[0065] In some embodiments, the tin disulfide nanosheets may include metal doping elements. Metal ion doping can expand the interlayer spacing of the tin disulfide nanosheets and provide more defects, thereby providing more active sites, promoting electron transport and the diffusion of active ions, and improving the capacity and cycling performance of the battery.

[0066] Exemplarily, the metal doping elements may include at least one of tungsten, vanadium, and niobium.

[0067] In some embodiments, the carbon coating layer may include a nitrogen-doped carbon coating layer. Nitrogen doping can further improve the reactivity and conductivity of the carbon coating layer. Moreover, the nitrogen atoms in the nitrogen-doped carbon have lone pairs of electrons, which can act as coordination sites to bind to tin ions, forming Sn-N coordination bonds, and can strengthen the chemical bond binding between the carbon coating layer and the tin disulfide nanosheets, thereby further enhancing the connection tightness and stability between the carbon coating layer and the tin disulfide nanosheets, and further improving the stability and electrochemical performance of the cathode material.

[0068] In some embodiments, the particle size of the core may be 0.5 μm to 3 μm, for example, it may be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or any value between any two of the above numerical ranges. Controlling the particle size of the core within the above range is beneficial to forming a relatively complete and high-quality coating layer on the surface of the core, which is further beneficial for the tin disulfide nanosheets to bind well to the carbon coating layer through chemical bonds, and makes the finally formed cathode material have high structural stability and cycling stability.

[0069] It can be understood that when the thickness of the coating layer is too thin, it is not conducive to forming a complete and uniform coating, and the effect of suppressing the deformation of the core and improving the conductivity of the cathode material through the coating layer is limited; when the thickness of the coating layer is too thick, the diffusion distance of active ions will increase. Therefore, in some embodiments, the thickness of the coating layer may be 3 nm to 20 nm, for example, it may be 3 nm, 5 nm, 7 nm, 10 nm, 12 nm, 15 nm, 18 nm, 20 nm, or any value between any two of the above numerical ranges. In this way, it is beneficial to form a complete and high-quality coating, better ensuring that the coating layer can fully play the role of suppressing the deformation of the core during battery cycling and improving the conductivity of the cathode material, while reducing the impact on the diffusion of active ions.

[0070] In some embodiments, the mass ratio of tin disulfide nanosheets to the core can be from 1:2 to 1:10, for example, it can be 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10 or any value between any two of the above numerical ranges. This can enable the tin disulfide nanosheets to more comprehensively cover the coating layer, thereby further increasing the specific surface area of the cathode material, providing more active sites, further accelerating the infiltration and penetration of the electrolyte, and further improving the capacity and cycling performance of the cathode material through the cooperation of the tin disulfide nanosheets and the iron disulfide core.

[0071] In some embodiments, the thickness of the tin disulfide nanosheets can be from 10 nm to 50 nm, for example, it can be 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm or any value between any two of the above numerical ranges. Controlling the thickness of the tin disulfide nanosheets within the above range can not only facilitate the formation of a tight and stable connection between the tin disulfide nanosheets and the carbon coating layer, but also better avoid the problem of agglomeration of the tin disulfide nanosheets, thereby being able to better increase the specific surface area of the cathode material, better promote the penetration of the electrolyte, as well as the transmission of electrons and active ions, provide more active sites, and thus better improve the capacity and cycling performance of the battery.

[0072] The embodiments of the present application also provide a preparation method of a cathode material. Please refer to Figure 1 , the preparation method of the cathode material provided by the embodiments of the present application includes the following steps:

[0073] S1: Add a tin source, a first sulfur source, and iron disulfide with a coating layer to a mixed solution of water and alcohol, carry out a first reaction to form tin disulfide nanosheets on the coating layer, and after washing and drying, obtain the cathode material; the coating layer includes a carbon coating layer, and the carbon coating layer is chemically bonded to the tin disulfide nanosheets.

[0074] In the embodiments of the present application, the coating layer on the surface of iron disulfide includes a carbon coating layer, which can effectively inhibit the deformation of iron disulfide during the battery cycling process and can also better improve the conductivity of the cathode material; by forming tin disulfide nanosheets on the coating layer, the sulfur vacancies on the surface of the tin disulfide nanosheets can form C-S bonds with carbon, enabling the tin disulfide nanosheets to be tightly bonded to the carbon coating layer through chemical bonds. The tin disulfide nanosheets cover at least part of the coating layer, significantly increasing the specific surface area of the cathode material, being able to accelerate the penetration of the electrolyte, provide more active sites, shorten the electron transport distance, and promote the diffusion of active ions. Moreover, tin disulfide has a significantly higher specific capacity than iron disulfide, so it can better improve the capacity and cycling performance of the battery.

[0075] In some embodiments, for step S1, while adding a tin source, a first sulfur source, and iron disulfide with a coating layer to a mixed solution of water and alcohol, a metal salt can also be added to the mixed solution so that the formed tin disulfide nanosheets include metal doping elements. Metal ion doping can expand the interlayer spacing of the tin disulfide nanosheets and provide more defects, thereby being able to provide more active sites, promoting the transport of electrons and the diffusion of active ions, and improving the capacity and cycling performance of the battery.

[0076] Exemplarily, the metal salt includes at least one of ammonium metatungstate, sodium metavanadate, and niobium pentachloride.

[0077] For step S1, in the actual preparation process, a tin source, a first sulfur source, and iron disulfide with a coating layer can be added to a mixed solution of water and alcohol, and then transferred to a reaction kettle for a first reaction (which can also be called a solvothermal reaction). After the reaction is completed, the reaction product is washed alternately with deionized water and alcohol, and then dried to obtain the positive electrode material.

[0078] There are no special restrictions on the types of the tin source and the first sulfur source in step S1, and they can be selected according to actual needs.

[0079] The "tin source" refers to a compound used to provide tin. Exemplarily, the tin source can include tin chloride and / or tin oxalate.

[0080] The "first sulfur source" refers to a compound used to provide sulfur. Exemplarily, the first sulfur source can include at least one of sulfur powder, thioacetamide, and thiourea.

[0081] In step S1, in the mixed solution of water and alcohol, the volume ratio of water to alcohol can be 1:15 to 15:1. This can better promote the progress of the first reaction.

[0082] In step S1, the temperature of the first reaction can be 160°C to 180°C, for example, it can be 160°C, 165°C, 170°C, 175°C, 180°C, or any value between any two of the above numerical ranges. This can ensure that the first reaction proceeds more fully and efficiently, is conducive to growing tin disulfide nanosheets with higher quality and better coverage on the coating layer, and thus further improves the comprehensive performance of the finally prepared positive electrode material.

[0083] Furthermore, the time of the first reaction can be 4h to 12h, for example, it can be 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, or any value between any two of the above numerical ranges. This can make the first reaction proceed more fully and completely, and thus better improve the comprehensive performance of the finally prepared positive electrode material.

[0084] In step S1, the drying temperature can be 60°C to 80°C. For example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, or any value between any two of the above numerical ranges. In this way, the washed reaction product can be fully dried, while avoiding the influence of too high temperature on the quality and performance of the obtained cathode material.

[0085] It can be understood that by adjusting the preparation process parameters in step S1, the morphology of the tin disulfide nanosheets grown on the coating layer can be controlled. In some specific embodiments, the thickness of the formed tin disulfide nanosheets can be controlled within the range of 10 nm to 50 nm. In this way, not only can a tight and stable connection be formed between the tin disulfide nanosheets and the carbon coating layer, but also the problem of agglomeration of the tin disulfide nanosheets can be better avoided, so as to better improve the specific surface area of the obtained cathode material, better promote the penetration of the electrolyte and the transport of electrons and active ions, provide more active sites, and thus better improve the capacity and cycle performance of the battery.

[0086] In some embodiments, please refer to Figure 2 , the preparation method of the iron disulfide with a coating layer in step S1 may include the following steps:

[0087] S11: Dissolve dopamine hydrochloride in an alkaline solution, mix evenly to obtain a mixed solution;

[0088] S12: Add iron disulfide to the mixed solution, stir to form a polydopamine coating layer on the surface of the iron disulfide, and dry to obtain a reaction product;

[0089] S13: Under the atmosphere of a protective gas, perform carbonization treatment on the reaction product to obtain iron disulfide with a coating layer (which can also be called a carbon-coated FeS2 microsphere).

[0090] Since dopamine has nitrogen-containing groups and can provide abundant functional groups (such as amino groups and phenolic hydroxyl groups), a polydopamine coating layer formed on the surface of iron disulfide by self-polymerization of dopamine is rich in active groups. After carbonizing the polydopamine coating layer, the obtained coating layer is a nitrogen-doped carbon coating layer, which can not only improve the reaction activity of the carbon coating layer, but also improve the conductivity of the carbon coating layer. At the same time, it also increases the specific surface area and pore volume of the material, which is beneficial to the penetration of the electrolyte and the exposure of active sites. Moreover, nitrogen atoms have lone pairs of electrons and can act as coordination sites to bind with tin ions to form Sn-N coordination bonds, which can strengthen the chemical bond binding effect between the carbon coating layer and the tin disulfide nanosheets, thereby promoting the growth of tin disulfide nanosheets on the surface of the coating layer, enhancing the connection tightness and stability between the carbon coating layer and the tin disulfide nanosheets, and further improving the stability and electrochemical performance of the finally prepared cathode material. In addition, the nitrogen-doped carbon coating layer formed through the polydopamine coating layer has active groups on its surface that can endow the material surface with more characteristics. For example, other polymers can be compounded on the surface by grafting and other methods, which is convenient for further modification of the material.

[0091] It can be understood that by controlling the mass ratio of dopamine hydrochloride to iron disulfide, the thickness of the finally formed carbon coating layer can be controlled. In some specific embodiments, the mass ratio of dopamine hydrochloride to iron disulfide can be 1:1 to 2:1, for example, it can be 1:1, 1.5:1, 2:1 or any value between any two of the above numerical ranges. This is beneficial to form a more complete and uniform coating on the surface of iron disulfide.

[0092] In the actual preparation process, the thickness of the coating layer can be controlled within the range of 3 nm to 20 nm. This is beneficial to form a complete and high-quality coating, better ensuring that the coating layer can fully play the role of suppressing the deformation of the core during the battery cycle and improving the conductivity of the cathode material, while reducing the influence on the diffusion of active ions.

[0093] There is no special limitation on the alkaline solution in the above step S11, as long as it can provide an alkaline environment so that dopamine can self-polymerize to form a polydopamine coating layer on the surface of iron disulfide. Exemplarily, the alkaline solution can include ammonia water and / or sodium hydroxide solution.

[0094] In the above step S12, the stirring time can be 24 h to 48 h, for example, it can be 24 h, 28 h, 32 h, 36 h, 40 h, 44 h, 48 h or any value between any two of the above numerical ranges. This can ensure that dopamine undergoes a relatively complete self-polymerization reaction to form a more complete and uniform polydopamine coating layer on the surface of iron disulfide.

[0095] In the above step S13, the protective gas can include at least one of nitrogen, argon, and helium.

[0096] In the above step S13, the temperature of the carbonization treatment can be 350°C to 600°C. For example, it can be 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, or any value between any two of the above numerical ranges. Controlling the temperature of the carbonization treatment within the above range is beneficial to fully carbonize the polydopamine coating layer, improve the quality of the prepared carbon coating layer, and at the same time avoid the influence of too high temperature on the performance of the material itself, so that the prepared iron disulfide with a coating layer has higher quality and performance.

[0097] Furthermore, the time of the carbonization treatment can be less than or equal to 10 h. In this way, it is possible to avoid the influence of too long carbonization treatment time on the performance of the material itself, and thus affect the performance of the finally prepared cathode material. Specifically, the shortest time for the carbonization treatment can be adjusted in combination with the thickness of the polydopamine coating layer, the temperature of the carbonization treatment, etc.

[0098] In some specific embodiments, the heating rate of the carbonization treatment can be 0.5°C / min to 20°C / min. For example, it can be 0.5°C / min, 5°C / min, 10°C / min, 15°C / min, 20°C / min, or any value between any two of the above numerical ranges. In this way, during the carbonization treatment, the material is heated more evenly, that is, the internal temperature distribution of the material is relatively uniform, which is beneficial to further improve the quality of the formed carbon coating layer, and thus further improve the quality of the finally prepared cathode material.

[0099] In some specific embodiments, the cooling rate of the carbonization treatment can be 0.5°C / min to 50°C / min. For example, it can be 0.5°C / min, 10°C / min, 20°C / min, 30°C / min, 40°C / min, 50°C / min, or any value between any two of the above numerical ranges. Controlling the cooling rate of the carbonization treatment within the above range can avoid the situation of thermal stress inside the material caused by rapid cooling, and thus avoid the cracking of the carbon coating layer and the partial detachment of the carbon coating layer from the surface of the iron disulfide.

[0100] Of course, the above method for preparing iron disulfide with a coating layer is only an example, and this application does not exclude the case of using other methods to prepare iron disulfide with a coating layer.

[0101] The preparation method of iron disulfide in any of the above embodiments may include: adding an iron source, a second sulfur source, and urea to a solvent, stirring evenly and then performing a second reaction, and washing and drying the reaction product to obtain iron disulfide.

[0102] In the above preparation method, there are no special restrictions on the types of iron source, second sulfur source and solvent, and they can be selected according to actual needs.

[0103] The "iron source" refers to a compound used to provide iron. Exemplarily, the iron source may include at least one of iron sulfate, iron nitrate, and iron chloride.

[0104] The "second sulfur source" refers to a compound used to provide sulfur. Exemplarily, the second sulfur source may include at least one of sulfur powder, thioacetamide, and thiourea.

[0105] Exemplarily, the solvent may include at least one of deionized water, ethanol, ethylene glycol, and N,N-dimethylformamide.

[0106] In some specific embodiments, the molar ratio of the iron source to the second sulfur source may be 1:3 to 1:10, for example, it may be 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, or any value between any two of the above numerical ranges. This is beneficial to improving the quality of the prepared iron disulfide.

[0107] In some specific embodiments, the molar ratio of the iron source to urea may be 1:5 to 1:10, for example, it may be 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, or any value between any two of the above numerical ranges. Urea can play a role such as complexing, promoting the formation of iron disulfide. Controlling the molar ratio of the iron source to urea within the above range is beneficial to improving the efficiency of preparing iron disulfide and the quality of the prepared iron disulfide.

[0108] In some specific embodiments, the temperature of the second reaction may be 150°C to 200°C, for example, it may be 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, or any value between any two of the above numerical ranges. This can ensure that the second reaction proceeds more fully and efficiently, which is beneficial to improving the efficiency of preparing iron disulfide and the quality of the prepared iron disulfide.

[0109] Furthermore, the time of the second reaction may be 12h to 48h, for example, it may be 12h, 16h, 20h, 24h, 28h, 32h, 36h, 40h, 44h, 48h, or any value between any two of the above numerical ranges. This can make the second reaction proceed more fully and completely, thereby better improving the quality of the prepared iron disulfide.

[0110] In the actual preparation process, an iron source, a second sulfur source, and urea can be placed into a reaction kettle, a solvent is added to the reaction kettle, and then stirred evenly to form a mixed solution. Then, the reaction kettle is placed in an oven for a second reaction to obtain a black precipitate product. After the reaction kettle is cooled to room temperature, the black precipitate product is taken out and washed alternately with ethanol and deionized water. Then, the washed product is placed in an oven to be dried, and a black powder sample of iron disulfide is obtained.

[0111] In the actual preparation process, the particle size of iron disulfide can be controlled within the range of 0.5 μm to 3 μm, which is beneficial to forming a relatively complete and high-quality coating layer on the surface of iron disulfide subsequently. Furthermore, it is beneficial to grow tin disulfide nanosheets with strong bonding, high quality, and good coverage on the coating layer, so that the finally formed cathode material has high structural stability and cycling stability.

[0112] Of course, the above method for preparing iron disulfide is only an example, and this application does not exclude the case of using other methods to prepare iron disulfide.

[0113] The embodiment of this application also provides a secondary battery, which includes a cathode plate. The cathode plate includes the cathode material described in any one of the above embodiments or the cathode material prepared by the preparation method of the cathode material described in any one of the above embodiments.

[0114] In some embodiments, the cathode plate may include a cathode current collector and a cathode active material layer provided on at least one surface of the cathode current collector. The cathode active material layer may include the cathode material described in any one of the above embodiments or the cathode material prepared by the preparation method of the cathode material described in any one of the above embodiments. As an example, the cathode current collector has two surfaces opposite to each other in its own thickness direction, and the cathode active material layer may be provided on any one or both of the two opposite surfaces of the cathode current collector.

[0115] It should be understood that the beneficial effects of the cathode material described in any one of the above embodiments or the cathode material prepared by the preparation method of the cathode material described in any one of the above embodiments are applicable to this secondary battery.

[0116] In some embodiments, the secondary battery may be a lithium-ion battery, a sodium-ion battery, or a magnesium-ion battery. Further, the cathode material described in any one of the above embodiments or the cathode material prepared by the preparation method of the cathode material described in any one of the above embodiments is particularly applicable to magnesium-ion batteries to solve the problem of restricting the rapid development of magnesium-ion batteries due to the lack of suitable cathode materials currently.

[0117] The technical solution of this application will be further described below in combination with multiple embodiments and comparative examples.

[0118] Example 1

[0119] In this embodiment, the preparation of the cathode material includes the following steps:

[0120] Step S101: Add 0.1 g of ferrous sulfate heptahydrate (iron source), 0.08 g of sulfur powder (second sulfur source), and 0.12 g of urea into the reaction kettle. Then add 6 mL of N,N-dimethylformamide and 8 mL of ethylene glycol (solvent) into the reaction kettle. After stirring evenly, place the reaction kettle in an oven at 180 °C for reaction for 12 h (second reaction) to obtain a black precipitate. After the reaction kettle is cooled to room temperature, take out the black precipitate, and wash the black precipitate alternately with ethanol and deionized water. Then place the washed black product in an oven at 70 °C for drying to obtain a black powder sample FeS2;

[0121] Step S102: Dissolve 0.5 g of dopamine hydrochloride in an ammonia water solution, stir evenly to obtain a mixed solution. Add the FeS2 prepared in step S101 into the mixed solution, stir and mix for reaction for 40 h to form a polydopamine coating layer on the surface of FeS2. After centrifugation and drying, obtain the reaction product. Next, calcine (carbonization treatment) under an argon atmosphere to obtain FeS2 microspheres coated with a nitrogen-doped carbon coating layer (iron disulfide with a coating layer), where the heating rate of the calcination is 2 °C / min, the temperature is 500 °C, and the time is 2 h;

[0122] Step S103: Add 0.5 g of stannous chloride dihydrate (tin source), 0.038 g of thiourea (first sulfur source), 0.15 g of ammonium metatungstate (metal salt), and 0.5 g of the FeS2 microspheres coated with a nitrogen-doped carbon coating layer prepared in step S102 into a mixed solution of 8 mL of water and 8 mL of ethanol. Transfer the evenly mixed solution to a reaction kettle at 180 °C for reaction for 12 h (first reaction). After the reaction is completed, take out the reaction product, and wash the reaction product alternately with deionized water and ethanol. Then dry the washed reaction product in an oven at 70 °C to obtain a composite material of carbon-coated iron disulfide microspheres loaded with metal ion-doped tin disulfide nanosheets (cathode material).

[0123] Example 2

[0124] The preparation of the cathode material in this embodiment is basically the same as that in Example 1, the differences are as follows:

[0125] 1) In step S102, the reaction time of stirring and mixing is adjusted from 40 h to 24 h;

[0126] 2) In step S103, the metal salt is sodium metavanadate, and the addition amount of sodium metavanadate is 0.006 g.

[0127] Example 3

[0128] In this embodiment, the preparation of the cathode material is basically the same as that in Example 1, except that:

[0129] 1) In step S102, the stirring and mixing reaction time is adjusted from 40 h to 24 h;

[0130] 2) In step S103, niobium pentachloride is used as the metal salt, and the addition amount of niobium pentachloride is 0.015 g.

[0131] Comparative Example 1

[0132] The preparation of the cathode material in this comparative example includes the following steps:

[0133] S201: Add 0.5 g of stannous chloride dihydrate and 0.038 g of thiourea to a mixed solution of 8 mL of water and 8 mL of ethylene glycol, transfer the uniformly mixed solution to a reaction kettle at 180 °C for reaction for 6 h. After the reaction is completed, take out the reaction product, and alternately wash the reaction product with deionized water and ethanol, and then dry the washed reaction product in an oven at 70 °C to obtain tin disulfide nanosheets (i.e., the cathode material).

[0134] Comparative Example 2

[0135] The preparation of the cathode material in this comparative example is the same as step S101 in Example 1, that is, the prepared FeS2 is directly used as the cathode material in this comparative example.

[0136] Comparative Example 3

[0137] The preparation of the cathode material in this comparative example includes the following steps:

[0138] Step S301: Add 0.1 g of ferrous sulfate heptahydrate (iron source), 0.08 g of sulfur powder (second sulfur source) and 0.12 g of urea into the reaction kettle, then add 6 mL of N,N-dimethylformamide and 8 mL of ethylene glycol (solvent) into the reaction kettle, stir evenly, place the reaction kettle in an oven at 180 °C for reaction for 12 h (the second reaction) to obtain a black precipitate. After the reaction kettle is cooled to room temperature, take out the black precipitate, and alternately wash the black precipitate with ethanol and deionized water, and then dry the washed black product in an oven at 70 °C to obtain a black powder sample FeS2;

[0139] Step S302: Add 0.5 g of stannous chloride dihydrate, 0.038 g of thiourea, and 0.5 g of FeS2 prepared in Step S301 into a mixed solution of 8 mL of water and 8 mL of ethanol. Transfer the uniformly mixed solution to a reaction kettle at 180 °C and react for 12 h. After the reaction is completed, take out the reaction product, and wash the reaction product alternately with deionized water and ethanol. Then, dry the washed reaction product in an oven at 70 °C to obtain a composite material of tin disulfide nanosheets supported on iron disulfide microspheres (i.e., the positive electrode material).

[0140] Prepare magnesium-ion batteries with the positive electrode materials in the above-mentioned examples and comparative examples, and test the electrochemical performance of the magnesium-ion batteries.

[0141] The preparation of the magnesium-ion battery includes the following steps:

[0142] (1) Prepare the positive electrode sheet: Add the positive electrode materials, conductive agents, and positive electrode binders prepared in the above-mentioned examples and comparative examples into a mortar according to a mass ratio of 8:1:1. After sufficient grinding, add an organic solvent, stir evenly, coat it on a copper foil, and form an active material layer after drying. Cut it into a disc with a diameter of 8 mm as the positive electrode sheet. Among them, the mass of the active material layer loaded on each copper foil is 0.5 mg;

[0143] (2) Prepare the negative electrode sheet: Polish the magnesium sheet smoothly and cut it into a disc with a diameter of 10 mm as the negative electrode sheet;

[0144] (3) Prepare the separator: Cut a glass fiber separator into a disc with a diameter of 19 mm as the separator;

[0145] (4) Prepare the filler: Cut a nickel foam into a disc with a diameter of 13 mm;

[0146] (5) Prepare the electrolyte: Weigh 1.066 g of anhydrous aluminum chloride and add it to 15 mL of tetrahydrofuran solution, stir continuously for 12 h, then use a pipette to dropwise add 5 mL of phenylmagnesium chloride solution (2 M, PhMgCl / THF), and stir continuously for 12 h to obtain a 0.4 M 2PhMgCl / THF-AlCl3 (abbreviated as APC) electrolyte. The entire preparation process is carried out in a glove box;

[0147] (6) Assembly of the magnesium-ion battery: Inside the glove box, place the positive electrode sheet prepared in step (1) in the battery case. After stacking the separator prepared in step (3) on the positive electrode sheet, drop the electrolyte prepared in step (5), then stack the negative electrode sheet prepared in step (2) on the separator, and then add the filler prepared in step (4). Use a sealer to encapsulate the battery to obtain a magnesium-ion battery.

[0148] The electrochemical performance of the above-prepared magnesium-ion battery was tested as follows:

[0149] (1) Tests of the first discharge capacity, first charge capacity, and first Coulombic efficiency: Using a battery test system, at 25 °C, first, a constant current discharge was carried out at a current density of 0.05 A / g until 0.02 V, then left standing for 5 minutes, and then charged at a current density of 0.05 A / g to 2.0 V to obtain the first charge capacity C1; then left standing for 5 minutes again, and a constant current discharge was carried out at a current density of 0.05 A / g until 0.02 V to obtain the first discharge capacity C2; the first Coulombic efficiency = (C2 / C1) * 100%.

[0150] (2) Test of the capacity retention rate after 50 cycles: At 25 °C, first, a constant current charge was carried out at a current density of 0.2 A / g to 2.0 V, then left standing for 5 minutes, and then a constant current discharge was carried out at a current density of 0.2 A / g until 0.02 V; cyclic tests were carried out according to this process, and the discharge capacities of the first cycle and the 50th cycle were recorded. By dividing the discharge capacity of the 50th cycle by the discharge capacity of the first cycle, the capacity retention rate after 50 cycles was obtained.

[0151] The test results are shown in Table 1.

[0152] Table 1

[0153]

[0154] It can be seen from the data in Table 1 that compared with Comparative Example 1 and Comparative Example 2, the first charge capacity per gram, the first discharge capacity per gram, and the first Coulombic efficiency of the batteries containing the cathode materials prepared in Examples 1 to 3 are all improved. At the same time, the capacity retention rate of the batteries after 50 cycles is higher than that of the batteries containing the cathode materials prepared in Comparative Example 2 and Comparative Example 3 after 50 cycles. This shows that in this application, a carbon coating layer is formed on the surface of iron disulfide. The carbon coating layer can effectively inhibit the deformation of iron disulfide during the battery cycle process, and can also better improve the conductivity of the cathode material. By forming tin disulfide nanosheets that at least cover part of the coating layer, the tin disulfide nanosheets can be tightly combined with the carbon coating layer through chemical bonds, significantly increasing the specific surface area of the cathode material, accelerating the penetration of the electrolyte, providing more active sites, shortening the electron transport distance, and promoting the diffusion of active ions. Moreover, tin disulfide has a significantly higher specific capacity than iron disulfide. Therefore, it can better improve the capacity and cycle performance of the battery.

[0155] The cathode material prepared in Comparative Example 1 is tin disulfide nanosheets. It can be seen from the data in Table 1 that although the capacity retention rate of the battery containing the cathode material prepared in Comparative Example 1 is at a relatively high level after 50 cycles, the first charge specific capacity, the first discharge specific capacity, and the first Coulombic efficiency of the battery are all relatively low, and the comprehensive performance of the battery is poor. The cathode material prepared in Comparative Example 2 is FeS2. It can be seen from the data in Table 1 that compared with Examples 1 to 3, the first charge specific capacity, the first discharge specific capacity, the first Coulombic efficiency, and the capacity retention rate after 50 cycles of the battery containing the cathode material prepared in Comparative Example 2 are all lower. It can be seen from this that it is difficult to improve the comprehensive performance of the battery by using tin disulfide nanosheets and FeS2 alone as the cathode material.

[0156] The cathode material prepared in Comparative Example 3 is a composite material of tin disulfide nanosheets loaded on iron disulfide microspheres. It can be seen from the data in Table 1 that although the first charge specific capacity, the first discharge specific capacity, and the first Coulombic efficiency of the battery containing the cathode material prepared in Comparative Example 3 are improved compared with Comparative Example 1 and Comparative Example 2 by the combination of iron disulfide and tin disulfide nanosheets, however, due to the lack of carbon coating on iron disulfide, the capacity retention rate of the corresponding battery after 50 cycles is relatively low, that is, the cycling performance of the battery is poor. This shows that it is difficult to effectively improve the cycling stability of the cathode material by directly combining iron disulfide and tin disulfide nanosheets without carbon coating on iron disulfide.

[0157] Figure 3 Figure Figure 3 is the scanning electron microscope image of the cathode material prepared in Example 1. It can be seen that the overall morphology of the cathode material prepared in Example 1 is spherical, and the tin disulfide nanosheets on the outermost layer of the cathode material have obvious sheet structures, growing relatively completely and uniformly on the carbon coating layer, forming an open framework structure, which significantly increases the specific surface area of the cathode material, can accelerate the penetration of the electrolyte, provide more active sites, shorten the electron transport distance, and promote the diffusion of active ions. Therefore, it can better improve the capacity and cycling performance of the battery.

[0158] It should be noted that the cathode material examples, the preparation method examples of the cathode material, and the secondary battery examples provided in this application belong to the same concept; among the technical features in the technical solutions recorded in each example, they can be combined arbitrarily without conflict.

[0159] It should be understood that the above embodiments are all exemplary and do not cover all possible implementation manners of the present application. Without departing from the scope of the present disclosure, various deformations and changes can also be made on the basis of the above embodiments. Similarly, various technical features of the above embodiments can also be arbitrarily combined to form additional embodiments of the present invention that may not be clearly described. Therefore, the above embodiments only represent several implementation manners of the present invention and do not limit the protection scope of the present invention patent.

Claims

1. A cathode material, characterized in that, It includes a core, a coating layer disposed on the surface of the core, and tin disulfide nanosheets covering at least a part of the coating layer; The core includes an iron disulfide core, the coating layer includes a carbon coating layer, and the carbon coating layer is chemically bonded to the tin disulfide nanosheets.

2. The cathode material according to claim 1, characterized in that, The positive electrode material satisfies at least one of the following characteristics: (1) The tin disulfide nanosheets include metal doping elements; optionally, the metal doping elements include at least one of tungsten, vanadium, and niobium; (2) The carbon coating layer includes a nitrogen-doped carbon coating layer; (3) The particle size of the core is 0.5 μm to 3 μm; (4) The thickness of the coating layer is 3 nm to 20 nm; (5) The mass ratio of the tin disulfide nanosheets to the core is 1:2 to 1:10; (6) The thickness of the tin disulfide nanosheets is 10 nm to 50 nm.

3. A method for preparing a cathode material, characterized in that, The method includes the following steps: S1: Add a tin source, a first sulfur source, and iron disulfide with a coating layer to a mixed solution of water and alcohol, carry out a first reaction to form tin disulfide nanosheets on the coating layer, and after washing and drying, obtain the positive electrode material; the coating layer includes a carbon coating layer, and the carbon coating layer is chemically bonded to the tin disulfide nanosheets.

4. The method for preparing the cathode material according to claim 3, wherein, In step S1, a metal salt is further added to the mixed solution, and the formed tin disulfide nanosheets include metal doping elements; optionally, the metal salt includes at least one of ammonium metatungstate, sodium metavanadate, and niobium pentachloride.

5. The method for preparing the cathode material according to claim 3, characterized in that, Step S1 satisfies at least one of the following characteristics: (1) The tin source includes tin chloride and / or tin oxalate; (2) The first sulfur source includes at least one of sulfur powder, thioacetamide, and thiourea; (3) The volume ratio of water to alcohol in the mixed solution is 1:15 to 15:1; (4) The temperature of the first reaction is 160°C to 180°C; (5) The time of the first reaction is 4 h to 12 h; (6) The temperature of the drying is 60°C to 80°C.

6. The method for preparing the cathode material according to any one of claims 3 to 5, characterized in that, The preparation method of the iron disulfide with a coating layer includes: Dissolve dopamine hydrochloride in an alkaline solution, mix evenly to obtain a mixed solution; Add iron disulfide to the mixed solution, stir to form a polydopamine coating layer on the surface of the iron disulfide, and dry to obtain a reaction product; Under the atmosphere of a protective gas, perform carbonization treatment on the reaction product to obtain the iron disulfide with a coating layer.

7. The method for preparing the cathode material according to claim 6, wherein The method satisfies at least one of the following characteristics: (1) The mass ratio of dopamine hydrochloride to iron disulfide is 1:1 to 2:1; (2) The alkaline solution includes ammonia water and / or sodium hydroxide solution; (3) The stirring time is 24 h to 48 h; (4) The protective gas includes at least one of nitrogen, argon, and helium; (5) The temperature of the carbonization treatment is 350°C to 600°C; (6) The time of the carbonization treatment is less than or equal to 10 h; (7) The heating rate of the carbonization treatment is 0.5°C / min to 20°C / min; (8) The cooling rate of the carbonization treatment is 0.5°C / min to 50°C / min.

8. The preparation method of the cathode material according to claim 6, characterized in that, The preparation method of the iron disulfide includes: Add an iron source, a second sulfur source, and urea to a solvent. After stirring evenly, conduct a second reaction. After washing and drying the reaction product, the iron disulfide is obtained.

9. The method for preparing the cathode material according to claim 8, wherein The method satisfies at least one of the following characteristics: (1) The molar ratio of the iron source to the second sulfur source is 1:3 to 1:10; (2) The molar ratio of the iron source to the urea is 1:5 to 1:10; (3) The iron source includes at least one of iron sulfate, iron nitrate, and iron chloride; (4) The second sulfur source includes at least one of sulfur powder, thiourea, or thioacetamide; (5) The solvent includes at least one of deionized water, ethanol, ethylene glycol, and N,N-dimethylformamide; (6) The temperature of the second reaction is 150°C to 200°C; (7) The time of the second reaction is 12 h to 48 h.

10. A secondary battery, characterized in that, It includes a positive electrode sheet, and the positive electrode sheet includes the positive electrode material described in Claim 1 or 2, or the positive electrode material prepared by the preparation method of the positive electrode material described in any one of Claims 3 to 9.

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