Double-shell structure conversion type positive electrode material, preparation method thereof and electrochemical device

Abstract

A dual-shell structure conversion cathode material, its preparation method, and an electrochemical device are disclosed. The dual-shell structure conversion cathode material comprises a matrix material and an inner shell layer and an outer shell layer sequentially coating the surface of the matrix material. The matrix material is a conversion cathode material, the inner shell layer is a liquid metal, and the outer shell layer is a metal oxide. This dual-shell structure conversion cathode material exhibits ideal capacity density, rate performance, cycle stability, and cycle life, making it suitable for high-rate cycling conditions over a wide temperature range.

Description

Technical Field

[0001] This application relates to the field of battery materials technology, specifically to a double-shell structure conversion cathode material, its preparation method, and electrochemical device. Background Technology

[0002] In recent years, the extensive use of traditional energy sources such as fossil fuels has exacerbated environmental pollution problems. Against the backdrop of continuously rising global energy demand, the development of sustainable energy storage devices and technologies has become an urgent priority. Lithium-ion batteries, due to their high energy density and low cost, have been widely used in portable electronic products, electric vehicles, and hybrid vehicles. However, currently commercially available intercalated lithium-ion battery cathodes suffer from low energy density and rising raw material costs. Therefore, it is necessary to develop new, low-cost, high-energy-density lithium-ion battery cathode materials to meet future energy demands. Conversion cathode materials involve multiple electron transfer reactions during charge and discharge, resulting in higher theoretical capacity. Compared to traditional intercalated cathode materials, conversion cathode materials have higher abundance of crustal elements, lower cost, and are mostly environmentally friendly, thus being considered a highly promising new cathode material.

[0003] However, conversion cathode materials have poor rate performance and suffer from severe volume decay during short-term cycling. Summary of the Invention

[0004] In view of this, in order to solve at least one of the above technical problems, embodiments of this application provide a double-shell structure conversion cathode material.

[0005] In addition, this application also provides a method for preparing the aforementioned double-shell structure conversion cathode material, and an electrochemical device using the double-shell structure conversion cathode material.

[0006] This application provides a dual-shell structure conversion cathode material, which includes a matrix material and an inner shell layer and an outer shell layer sequentially covering the surface of the matrix material. The matrix material is a conversion cathode material, the inner shell layer is liquid metal, and the outer shell layer is a metal oxide.

[0007] In some possible embodiments, the mass ratio of the conversion cathode material to the liquid metal is 50:(1~10).

[0008] In some possible embodiments, the mass ratio of the conversion cathode material to the metal oxide is 50:(1~5); and / or

[0009] The thickness of the outer shell layer is 2nm~7nm.

[0010] In some possible embodiments, the metal oxide includes at least one of tin oxide, zinc oxide, titanium dioxide, silicon dioxide, zirconium oxide, aluminum oxide, cobalt tetroxide, tungsten oxide, and bismuth oxide.

[0011] In some possible embodiments, the liquid metal includes at least one of gallium, gallium-tin alloy, gallium-indium alloy, and gallium-indium-tin alloy.

[0012] In some possible embodiments, the conversion cathode material includes at least one of elemental sulfur, sulfides, transition metal halides, and transition metal halide / carbon composite materials.

[0013] This application also provides a method for preparing a double-shell structure conversion cathode material, including:

[0014] A mixture is obtained by mixing a matrix material, liquid metal, and a dispersion medium, wherein the matrix material is a conversion-type cathode material;

[0015] The mixture is physically dispersed at a temperature above the melting point of the liquid metal, causing the liquid metal to coat the surface of the matrix material to form an inner shell, thereby obtaining a cathode material intermediate; and

[0016] The cathode material intermediate is dispersed in a liquid medium containing metal elements, and a layer of metal oxide is synthesized in situ on the surface of the inner shell layer using a wet chemical synthesis method to form an outer shell layer, thereby obtaining the double-shell structure conversion cathode material.

[0017] In some possible embodiments, the liquid metal includes at least one of gallium, gallium-tin alloy, gallium-indium alloy, and gallium-indium-tin alloy; and / or

[0018] The mass ratio of the conversion-type cathode material to the liquid metal is 50:(1~10).

[0019] In some possible embodiments, the physical dispersion includes at least one of planetary ball milling, high-energy ball milling, ultrasonic dispersion, high-speed shear dispersion, high-speed grinding, mechanical stirring dispersion, and high-pressure homogenization dispersion; and / or

[0020] The wet chemical synthesis method includes at least one of the sol-gel method, hydrothermal method, and in-situ alcoholysis method.

[0021] In some possible embodiments, the wet chemical synthesis method is an in-situ alcoholysis method, wherein the reaction conditions of the in-situ alcoholysis method include: a reaction temperature of 30°C to 60°C, a reaction time of 24h to 48h, and the liquid medium includes organometallic salts and alcohols.

[0022] Additionally, this application also provides an electrochemical device, including a positive electrode sheet, wherein the positive electrode sheet includes a positive electrode material, and the positive electrode material is the aforementioned double-shell structure conversion type positive electrode material.

[0023] Compared to existing technologies, the dual-shell structure conversion cathode material provided in this application uses a conversion cathode material as the matrix material, liquid metal as the inner shell layer, and metal oxide as the outer shell layer. First, the liquid metal in the inner shell layer possesses "self-healing" properties. During charging and discharging, as the conversion cathode material undergoes volume changes, the liquid metal forms an interface layer with plastic deformation capabilities, mitigating the volume changes of the matrix material and inhibiting the dissolution of active substances in the matrix material, thereby effectively improving the capacity and cycle life of the dual-shell structure conversion cathode material. The liquid metal also provides electron and ion transport channels between matrix material particles, thereby improving the rate performance of the dual-shell structure conversion cathode material. Furthermore, the formation of a metal oxide outer shell layer on the surface of the liquid metal restricts large-scale movement and aggregation of the liquid metal, reducing localized aggregation and contributing to maintaining the stability and uniformity of the entire liquid metal inner shell layer. Through the synergistic effect of the inner and outer shell layers, the matrix material is doubly fixed and buffered, reducing the volume expansion of the matrix material and the loss of positive electrode active material, and improving the interfacial stability of the double-shell structure conversion cathode material. This allows the double-shell structure conversion cathode material to exhibit ideal capacity density, excellent cycle stability and cycle life, and is suitable for high-rate cycling conditions over a wide temperature range. Attached Figure Description

[0024] Figure 1 This is a process flow diagram of a method for preparing a double-shell structure conversion cathode material according to an embodiment of this application.

[0025] Figure 2 The images show the X-ray diffraction patterns of the double-shell FeF3 / carbon powder cathode material in Example 2 of this application, the FeF3 / carbon powder cathode material in Comparative Example 1, and the commercial FeF3 material.

[0026] Figure 3 This is a scanning electron microscope image of the double-shell FeF3 / carbon powder cathode material in Example 2 of this application.

[0027] Figure 4 This is an EDS elemental distribution analysis diagram of the double-shell structure FeF3 / carbon powder cathode material in Example 2 of this application, wherein, Figure 4 Figure a shows the morphology of the double-shell structure FeF3 / carbon powder cathode material. Figure 4 Figure b shows the distribution of element C. Figure 4 The c-plot shows the distribution of O elements. Figure 4 The d-plot represents the distribution of element F. Figure 4 The e-plot shows the distribution of Fe elements. Figure 4The f-plot shows the distribution of Ga elements. Figure 4 The g-plot shows the distribution of Ti elements.

[0028] Figure 5 This is a rate performance diagram of the battery prepared using the double-shell FeF3 / carbon powder cathode material in Example 2 of this application. Detailed Implementation

[0029] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0030] Conversion-type cathode materials suffer from poor rate performance and severe capacity decay, mainly because volume changes, unstable reaction interfaces, and dissolution of cathode active materials are prone to occur during charging and discharging.

[0031] Therefore, this application provides a dual-shell structure conversion cathode material, which includes a matrix material and an inner shell layer and an outer shell layer sequentially covering the surface of the matrix material. The matrix material is a conversion cathode material, the inner shell layer is liquid metal, and the outer shell layer is a metal oxide.

[0032] In this embodiment, by coating the surface of the substrate material with a liquid metal inner shell, firstly, due to the "self-healing" properties of liquid metal, it can effectively adapt to the volume and shape changes of the substrate material during charging and discharging, constructing a coating layer that is not easily damaged and has intact mechanical structural properties. This provides an interface layer capable of plastic deformation, effectively mitigating the problems of cathode material cracking and expansion caused by volume changes, and reducing mechanical damage to the double-shell structure conversion cathode material during charging and discharging. Secondly, the substrate material generates some soluble metal ion intermediates during charging and discharging. The liquid metal can spontaneously alloy with these soluble metal ion intermediates, fixing the soluble metal ions within the inner shell structure. This inhibits the dissolution of the active material in the cathode material during charging and discharging, thereby constructing a "dynamically stable" cathode interface and significantly improving the capacity density, cycle stability, and cycle life of the double-shell structure conversion cathode material. Furthermore, the liquid metal has good conductivity, and this inner shell layer, as a conductive network, can also provide electron and ion transport channels between substrate material particles, effectively improving the electrode capacity and cycle life.

[0033] This application further enhances the stability and uniformity of the entire double-shell structure by forming a metal oxide outer shell layer on the surface of the inner shell. Firstly, liquid metal is typically affected by electric fields, thermal effects, or volume expansion during battery charging and discharging, leading to displacement or flow. The metal oxide outer shell layer effectively "traps" the liquid metal within the inner shell, limiting its large-scale movement and accumulation during charging and discharging, thus reducing local instability or structural damage caused by the movement of the liquid metal. Secondly, the high degree of thermal expansion matching between the metal oxide outer shell layer and the liquid metal inner shell layer effectively reduces internal stress caused by temperature changes and structural deformation due to thermal expansion and contraction, maintaining the stability of the overall structure. The outer shell layer possesses good mechanical strength, corrosion resistance, thermal stability, and chemical stability, contributing to structural support and effectively reducing the cracking or damage to the inner shell layer due to expansion or contraction. It also protects the inner shell layer and substrate material from electrolyte or environmental corrosion, reducing structural damage caused by heat generated by high temperatures or electrochemical reactions, maintaining interface stability, and preserving the overall morphology of the cathode material. Furthermore, the metal oxide shell may form unique interfacial effects with the liquid metal during charging and discharging, such as interfacial tension, further restricting the movement and accumulation of the liquid metal. The metal oxide shell layer can also physically isolate the substrate material from the electrolyte, reducing the reaction between the positive electrode active material in the substrate material and the electrolyte, further reducing the loss of the positive electrode active material, thereby improving the specific capacity of the positive electrode material.

[0034] Therefore, this application achieves dual fixation and buffering of the matrix material through the synergistic effect of the inner and outer shell layers. The inner shell layer, capable of plastic deformation, directly absorbs and buffers the volume changes of the matrix material, provides conductive channels, and enhances reactivity, facilitating rapid electron and ion transport. The outer shell layer provides mechanical support for secondary fixation and protection, collectively reducing the volume expansion of the matrix material and improving the interfacial stability of the double-shell structure conversion cathode material. Furthermore, the double-shell structure effectively reduces the loss of active material in the cathode. The synergistic effect of the inner and outer shell layers enables the double-shell structure conversion cathode material to exhibit ideal capacity density, excellent cycle stability, and cycle life, making it suitable for high-rate cycling conditions over a wide temperature range.

[0035] Conversion-type cathode materials can include at least one of elemental sulfur, sulfides, transition metal halides, and transition metal halide / carbon composite materials, possessing advantages such as high theoretical capacity, abundant raw materials, low cost, and environmental friendliness. Further conversion-type cathode materials can be transition metal halide / carbon composite materials, and even further, metal fluoride / carbon composite materials, such as FeF3 / C composite materials or CuF2 / C composite materials. The carbon material can construct a continuous conductive network for the metal fluoride, improving the conductivity of the conversion-type cathode material. Specifically, the carbon material can be a carbon-based conductive additive, with the metal fluoride located in the gaps of the conductive structure formed by the carbon-based conductive additive material. Specifically, carbon-based conductive additives can include, but are not limited to, at least one of carbon nanotubes, Super-P, acetylene black, Ketjen black, XC-72 black, and ordered mesoporous carbon CMK-3.

[0036] The liquid metal can include at least one of gallium, gallium-tin alloy, gallium-indium alloy, and gallium-indium-tin alloy. These liquid metals possess excellent electrical conductivity, high thermal conductivity, and flexibility, which is beneficial for further improving the conductivity, safety, and cycle stability of the conversion-type cathode material. Furthermore, these liquid metal alloys have low melting points and good phase transition characteristics, allowing them to transform between solid and liquid states, providing more electrochemical reaction sites, thereby improving the battery's energy density and charge / discharge efficiency. The liquid metal can further be gallium.

[0037] The metal oxide may include at least one of tin oxide, zinc oxide, titanium dioxide, silicon dioxide, zirconium oxide, aluminum oxide, cobalt tetroxide, tungsten oxide, and bismuth oxide. These metal oxides possess high chemical stability, good mechanical strength, and excellent corrosion resistance, effectively extending the service life of dual-shell structure conversion cathode materials. Furthermore, these metal oxides exhibit excellent thermal stability and a high melting point, remaining stable even at high temperatures, reducing the risk of thermal runaway. The metal oxide may further be at least one of titanium dioxide and silicon dioxide.

[0038] In some embodiments, the mass ratio of the conversion cathode material to the liquid metal can be 50:(1~10). Adding an appropriate amount of liquid metal can form an effective and uniform inner shell layer on the material surface, further improving the energy density and cycle performance of the cathode material. Exemplarily, it can be any value within the range of 50:1, 50:2, 50:3, 50:4, 50:5, 50:6, 50:7, 50:8, 50:9, 50:10, or any two of the above values. This mass ratio can further be 50:(3~6).

[0039] In some embodiments, the mass ratio of the conversion cathode material to the metal oxide can be 50:(1~5). This range of mass ratios is beneficial for providing sufficient structural strength and chemical stability to the metal oxide shell layer, while also fully utilizing the high energy storage capacity of the conversion cathode material. Exemplarily, it can be any value within the range of 50:1, 50:2, 50:3, 50:4, 50:5, or any two of these values. This mass ratio can further be 50:(2~4).

[0040] In some embodiments, the thickness of the outer shell layer can be 2 nm to 7 nm, which can effectively restrict the large-scale movement and accumulation of liquid metal and effectively improve the structural and chemical stability of the double-shell structure conversion cathode material. The thickness of the outer shell layer can, exemplarily, be any value within the range of 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, or any two of the above values. The thickness of the outer shell layer can further be 3 nm to 5 nm.

[0041] Understandably, in addition to conversion-type cathode materials, the modification method using a liquid metal and metal oxide double shell in this application can be used for other lithium-ion battery electrode materials with large volume changes during charging and discharging, such as silicon-based anode materials, which can effectively alleviate capacity decay and electrode failure caused by volume expansion.

[0042] Compared with the prior art, the double-shell structure conversion cathode material provided in this application has the following beneficial effects:

[0043] 1. The dual-shell structure conversion cathode material provided in this application has an inner shell layer and an outer shell layer sequentially formed on the surface of the substrate material. The inner shell layer provides an interface layer with plastic deformation capability, effectively mitigating the volume expansion of the substrate material and inhibiting the dissolution of active substances in the substrate material. At the same time, the outer shell layer restricts the large-scale movement and aggregation of liquid metal, maintaining the stability and uniformity of the entire inner shell layer. The synergistic effect of the dual-shell structure provides dual fixation and buffering for the substrate material, enabling the dual-shell structure conversion cathode material to exhibit ideal capacity density, rate performance, cycle stability, and cycle life, making it suitable for high-rate cycling conditions over a wide temperature range.

[0044] 2. By adjusting the mass ratio of conversion cathode material, liquid metal and metal oxide in the double-shell structure conversion cathode material, it is beneficial to control the energy density, cycle performance and structural stability of the double-shell structure conversion cathode material to a high level.

[0045] Please see Figure 1 As shown, based on the same inventive concept, the preparation method of the double-shell structure conversion cathode material provided in this application specifically includes the following steps:

[0046] Step S1: Mix the matrix material, liquid metal and dispersion medium to obtain a mixture, wherein the matrix material is a conversion-type cathode material.

[0047] Specifically, in a glove box, a certain mass ratio of the conversion-type positive electrode material and liquid metal are added to a container, and an appropriate amount of dispersion medium is added to the container to obtain a mixture.

[0048] The conversion-type cathode material may include at least one of elemental sulfur, sulfides, transition metal halides, and transition metal halide / carbon composite materials. The conversion-type cathode material may further be a transition metal fluoride or a transition metal fluoride / carbon composite material.

[0049] The liquid metal may include at least one of gallium, gallium-tin alloy, gallium-indium alloy, and gallium-indium-tin alloy. The liquid metal may further be gallium.

[0050] In some embodiments, the mass ratio of the conversion cathode material to the liquid metal can be 50:(1~10). This mass ratio range is advantageous for coating an appropriate amount of liquid metal onto the surface of the conversion cathode material, forming a complete and continuous coating layer to suppress the dissolution of the cathode active material, while not affecting lithium-ion transport efficiency, thereby balancing the energy density and cycle performance of the dual-shell structure conversion cathode material. Exemplarily, it can be 50:1, 50:2, 50:3, 50:4, 50:5, 50:6, 50:7, 50:8, 50:9, 50:10, or any value within the range of any two of these values. This mass ratio can further be 50:(2~5).

[0051] The dispersion medium can be an organic solvent that does not react with the conversion cathode material and the liquid metal. The dispersion medium facilitates the uniform mixing and effective dispersion of the liquid metal and the conversion cathode material, while providing an inert environment to prevent reaction interference and improve the coating effect. For example, the dispersion medium can be a liquid dispersion medium, including at least one selected from anhydrous ethanol, anhydrous acetonitrile, N-methylpyrrolidone, acetone, N,N-dimethylformamide, isopropanol, ethylene glycol dimethyl ether, toluene, ethylene glycol, propylene glycol, and dimethyl sulfoxide. Further, the dispersion medium may include at least one selected from anhydrous ethanol, anhydrous acetonitrile, N-methylpyrrolidone, and acetone.

[0052] In some embodiments, the preparation of transition metal fluoride / carbon composite materials in conversion cathode materials, taking FeF3 / C and CuF2 / C as examples, can be as follows:

[0053] First, prepare an iron or copper source solution of a certain concentration, and add a certain amount of carbon source to the solution. At this time, ultrasonic technology can be used to promote dispersion and obtain a mixture. Then, after drying the mixture, heat it in a fluorine source gas atmosphere to fluorinate it, and the FeF3 / C or CuF2 / C composite material can be obtained.

[0054] Alternatively, FeF3 or CuF2 can be directly physically dispersed with a carbon source (e.g., ball milling) to obtain the FeF3 / C or CuF2 / C composite material.

[0055] Step S2: At a temperature higher than the melting point of the liquid metal, the mixture is physically dispersed so that the liquid metal coats the surface of the matrix material to form an inner shell layer, thereby obtaining a cathode material intermediate.

[0056] Specifically, after physically dispersing the mixture obtained in step S1 under conditions higher than the melting point of the liquid metal, the sample after the above physical dispersion treatment is transferred to a drying device and dried in a drying environment at a certain temperature, so that the liquid metal fully coats the surface of the matrix material particles to form an inner shell layer, thereby obtaining a positive electrode material intermediate with a liquid metal inner shell layer coating.

[0057] In this step, after physical dispersion and drying, liquid metal particles are uniformly coated on the outside of the substrate material. A certain physical adsorption force exists between the liquid metal particles and the substrate material surface, fixing the liquid metal particles to the substrate material surface to form a liquid metal coating layer, which is the inner shell layer in the double-shell structure conversion cathode material. The liquid metal can provide an interface layer capable of plastic deformation, effectively mitigating the volume expansion of the conversion cathode material. It can also provide electron and ion transport channels for the conversion cathode material, thereby improving the capacity and cycle life of the double-shell structure conversion cathode material.

[0058] In this step, physical dispersion is utilized, employing external shear and impact mechanical energy to break the liquid metal from macroscopic droplets into nanoscale microdroplets, ensuring their uniform distribution within the dispersion medium. Simultaneously, the mechanical force promotes full contact between the liquid metal microdroplets and the conversion cathode material particles, generating force interactions that ensure effective coating of the conversion cathode material particles by the liquid metal. Compared to chemical dispersion methods, physical dispersion methods are lower in cost and simpler to operate, making them more suitable for the preparation method of the double-shell structure conversion cathode material in this application.

[0059] In some embodiments, physical dispersion may include at least one of planetary ball milling, high-energy ball milling, ultrasonic dispersion, high-speed shear dispersion, high-speed grinding, mechanical stirring dispersion, and high-pressure homogenization dispersion. Physical dispersion may further include at least one of planetary ball milling, high-energy ball milling, ultrasonic dispersion, and high-speed shear dispersion. It is understood that physical dispersion includes, but is not limited to, the above-described methods; any method that achieves sufficient dispersion is acceptable.

[0060] In some embodiments, the drying conditions may include: a temperature of 70°C to 120°C, a time of 6 hours to 12 hours, and a vacuum environment. The drying equipment may be a vacuum oven. During the drying process, the liquid metal can spread and penetrate more uniformly to the surface of the matrix material particles, effectively removing any moisture or other volatile impurities that may be present in the mixture. The vacuum environment can also reduce the formation of bubbles, forming a continuous and complete inner shell layer, thereby improving the overall stability and electrochemical performance of the material. The temperature may, exemplarily, be any value within the range of any two of the values ​​composed of 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, or more specifically, 80°C to 100°C. The time may, exemplarily, be any value within the range of any two of the values ​​composed of 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, or more specifically, 8 hours to 10 hours.

[0061] Step S3: The cathode material intermediate is dispersed in a liquid medium containing metal elements, and a layer of metal oxide is synthesized in situ on the surface of the inner shell layer by a wet chemical synthesis method to form the outer shell layer, thereby obtaining a double-shell structure conversion cathode material.

[0062] Specifically, the cathode material intermediate is dispersed in a liquid medium containing metal elements, and a layer of metal oxide is synthesized in situ on the surface of the inner shell using a wet chemical synthesis method. Then, solid precipitation separation and drying are performed to obtain a double-shell structure conversion cathode material.

[0063] Among these methods, wet chemical synthesis enables in-situ synthesis of metal oxide layers under mild conditions, which is beneficial for controlling the thickness, uniformity, and tightness of the outer shell layer with the inner shell layer, as well as the composition and morphology of the metal oxides. This improves the structural stability and electrochemical performance of the double-shell structure conversion cathode material. Furthermore, compared to the high-temperature solid-state method, wet chemical synthesis is simpler to operate, consumes less energy, has lower costs, and is easier to scale up for mass production.

[0064] In some embodiments, wet chemical synthesis may include at least one of sol-gel method, hydrothermal method and in-situ alcoholysis method.

[0065] Furthermore, wet chemical synthesis can employ in-situ alcoholysis, where the liquid medium includes organometallic salts and alcohols. In-situ alcoholysis, under mild conditions, promotes the hydrolysis and condensation reactions of organometallic salts through the action of alcohol solvents, thereby uniformly growing a metal oxide layer, i.e., the outer shell layer, on the surface of the inner shell. In addition, the alcoholysis process helps to obtain a uniform and robust metal oxide layer, reducing the inhomogeneity or detachment problems of the outer shell layer, and also enhances the adhesion between the inner shell layer and the substrate material, improving the overall stability of the double-shell structure.

[0066] The reaction conditions for in-situ alcoholysis may include: (i) a reaction temperature of 30℃ to 60℃, which eliminates the need for high-temperature treatment, helps reduce energy consumption, and minimizes the adverse effects of high temperatures on the double-shell structure conversion cathode material, especially effectively protecting the stability of the liquid metal. The reaction temperature can, for example, be any value within the range of 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, or any two of the above values. The reaction temperature can further be 40℃ to 50℃. (ii) a reaction time of 24h to 48h, which is beneficial for the complete progress of the reaction. The reaction time can, for example, be any value within the range of 24h, 30h, 36h, 42h, 48h, or any two of the above values. The reaction time can further be 24h to 36h.

[0067] The liquid medium used includes organometallic salts and alcohols, and may further be a mixed solution containing only organometallic salts and alcohols. The organometallic salt may include at least one selected from tetrabutyl titanate, titanium isopropoxide, tetraethoxysilane, tetraethyl vanadate, triisobutylaluminum, and zirconium n-butoxide. The alcohol may include at least one selected from methanol, ethanol, isopropanol, butanol, cyclohexanol, glycerol, ethylene glycol, and propylene glycol.

[0068] The ratio of organometallic salt to cathode material intermediate in the liquid medium can be (0.1~0.4) mol:1g, which is beneficial for the formation of an appropriate amount of metal oxide. This ratio can be exemplarily 0.1 mol:1g, 0.2 mol:1g, 0.3 mol:1g, 0.4 mol:1g, or any value within the range of any two of these values. Further, this ratio can be (0.2~0.4) mol:1g.

[0069] In the aforementioned liquid medium, the ratio of organometallic salt to alcohol can be (0.05~0.1) mol:1L, which allows the formation rate of metal oxides to be controlled at a suitable level. Exemplarily, it can be 0.05 mol:1L, 0.06 mol:1L, 0.07 mol:1L, 0.08 mol:1L, 0.08 mol:1L, 0.1 mol:1L, or any value within the range of any two of these values. Further, this ratio can be (0.06~0.08) mol:1L.

[0070] In some embodiments, the drying conditions may include: a temperature of 70°C to 120°C, a time of 6 hours to 12 hours, and a vacuum environment. The drying equipment may be a vacuum oven. During the drying process, the aforementioned temperature and vacuum environment are beneficial for forming a continuous and complete outer shell layer, thereby improving the overall stability and electrochemical performance of the double-shell structure conversion cathode material. The temperature may, exemplarily, be any value within the range of any two of the values ​​composed of 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, or more specifically, 80°C to 100°C. The time may, exemplarily, be any value within the range of any two of the values ​​composed of 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, or more specifically, 8 hours to 10 hours.

[0071] Compared with the prior art, the preparation method of the double-shell structure conversion cathode material provided in this application has the following beneficial effects:

[0072] 1. A physical dispersion method is used to first coat the surface of the conversion cathode material with a layer of liquid metal, providing an inner shell layer with plastic deformation capability for the conversion cathode material, effectively mitigating the volume expansion of the conversion cathode material. Furthermore, the physical dispersion method has advantages such as high efficiency and low cost, making it suitable for the preparation method of this application.

[0073] 2. By employing a wet chemical synthesis method, a metal oxide outer shell layer is synthesized in situ on the surface of the inner shell. This helps reduce the large-scale movement and uneven aggregation of liquid metal in the inner shell, enhancing the structural and chemical stability of the double-shell structure conversion cathode material. Furthermore, the wet chemical synthesis method facilitates control of the metal oxide layer formation process, is simple to operate, has low energy consumption and low cost, and is easily achievable for large-scale production.

[0074] 3. This preparation method is simple, efficient, and inexpensive, which is conducive to the large-scale production of double-shell structure conversion cathode materials and has excellent commercialization prospects.

[0075] This application also provides an electrochemical device (e.g., a lithium-ion battery) comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode comprises a positive electrode material, which is a double-shell structure conversion-type positive electrode material as described above. The electrochemical device prepared using the aforementioned double-shell structure conversion-type positive electrode material maximizes the advantages of its high energy density while exhibiting good cycle stability and cycle life, making it suitable for applications in wide-temperature-range, high-rate cycling applications.

[0076] The negative electrode can be a lithium sheet. The electrolyte can be a locally high-concentration electrolyte, including lithium salts, ether solvents, and fluorinated diluents. For example, the lithium salt in the electrolyte is lithium bis(fluorosulfonyl)imide, the ether solvent is ethylene glycol dimethyl ether, and the fluorinated diluent is bis(2,2,2-trifluoroethyl) ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and fluorobenzene. The aforementioned wide temperature range can be between -20°C and 45°C, and the rate capability range is between 0.1°C and 10°C.

[0077] The following specific examples further illustrate the aforementioned double-shell structure conversion cathode material, its preparation method, and electrochemical device.

[0078] Example 1

[0079] Step S1: In a glove box, add 1000 mg of FeF2 / carbon composite material (conversion cathode material) and 100 mg of liquid gallium metal into a sealed ball mill jar, and add 10 mL of N-methylpyrrolidone to the ball mill jar to obtain a mixture.

[0080] Step S2: The mixture is ball-milled at 350 rpm for 5 hours to ensure that liquid gallium metal fully coats the surface of the FeF2 / carbon composite particles. The ball-milled slurry is then transferred to a vacuum oven, where air bubbles are slowly removed by vacuuming. After drying at 105°C for 8 hours, the liquid metal-coated FeF2 / carbon composite material is obtained, which is the cathode material intermediate.

[0081] Step S3: Disperse 100 mg of the cathode material intermediate in 20 mL of anhydrous ethanol, add 20 mmol of tetrabutyl titanate, and ultrasonically disperse in an ultrasonic cleaner for 30 minutes. Then, stir at 50 °C for 24 h to carry out in-situ alcoholysis reaction, forming a titanium dioxide coating layer in-situ on the surface of the cathode material intermediate. After centrifuging the above sample to separate the precipitate, transfer it to a vacuum oven and dry it in a drying environment at 105 °C for 8 h. After the sample is dried and cooled to room temperature, a double-shell structure conversion cathode material (double-shell structure FeF2 / carbon powder cathode material) is obtained and stored in a glove box environment.

[0082] Step S4: Use double-shell FeF2 / carbon powder material as the positive electrode material of lithium-ion battery, lithium sheet as the negative electrode material, and dissolve 10 mmol of bis(2,2,2-trifluoroethyl) ether in 10 mL of ethylene glycol dimethyl ether and 36 mL of bis(2,2,2-trifluoroethyl) ether solvent to form a secondary electrolyte, and assemble the battery.

[0083] Example 2

[0084] The difference from Example 1 is that in step S1, the conversion cathode material is selected as a FeF3 / carbon composite material. The other steps are basically the same as in Example 1; please refer to Example 1.

[0085] Example 3

[0086] The difference from Example 1 is that in step S1, the conversion cathode material is selected as a CuF2 / carbon composite material. The other steps are basically the same as in Example 1; please refer to Example 1.

[0087] Example 4

[0088] The difference from Example 2 is that in step S1, the mass of liquid gallium metal added is 50 mg; and in step S3, the amount of tetrabutyl titanate added is 40 mmol. The other steps are basically the same as in Example 2; please refer to Example 2.

[0089] Comparative Example 1

[0090] The same FeF3 / carbon composite material as in Example 2 was used, but without the double-shell structure. The FeF3 / carbon composite material was used as the positive electrode material for the lithium-ion battery, and lithium sheets were used as the negative electrode material. 10 mmol of bis(fluorosulfonyl)imide lithium salt was dissolved in 10 mL of ethylene glycol dimethyl ether and 36 mL of bis(2,2,2-trifluoroethyl) ether solvent as a secondary electrolyte, and the battery was assembled.

[0091] The following tests were performed on the double-shell structure conversion cathode materials and batteries obtained in Examples 1-4 and Comparative Example 1.

[0092] 1. X-ray diffraction test to observe the crystal structure and elemental composition of the cathode material.

[0093] 2. Use a scanning electron microscope to observe the form in which the cathode material exists.

[0094] 3. EDS mapping (energy dispersive X-ray spectroscopy imaging) is used to observe the surface morphology of the cathode material and the distribution of elements in the cathode material.

[0095] 4. High and low temperature rate performance test: The battery rate performance was tested sequentially using current densities of 0.5C, 1C, 2C, 5C and 0.5C. Each current density was cycled 10 times. 1C was defined as 719mAh / g. Rate cycling was performed at low temperature of 0℃, normal temperature of 25℃ and high temperature of 40℃.

[0096] The above test results show that:

[0097] like Figure 2 As shown, compared with commercial FeF3 material and Comparative Example 1, the X-ray diffraction results of the double-shell FeF3 / carbon cathode material prepared in Example 2 show that Example 2 maintains the crystal structure of FeF3. Gallium and TiO2 in the double-shell FeF3 / carbon cathode material exist in an amorphous state, which helps to alleviate the volume expansion of the FeF3 / carbon cathode material. Simultaneously, Example 2 was characterized by scanning electron microscopy and EDS mapping, and the test results are shown below. Figure 3 and Figure 4 As shown. By Figure 3 It can be seen that the double-shell FeF3 / carbon cathode material exists in the form of nanoparticles. (From...) Figure 4 It can be seen that the gallium element in the inner shell and the Ti element in the outer shell are uniformly distributed on the surface of FeF3 / carbon particles, indicating that the double-shell structure is uniformly coated on the surface of the conversion cathode material.

[0098] The rate performance of the coin cell using the double-shell FeF3 / carbon cathode material prepared in Example 2 is as follows: Figure 5 As shown, the high and low temperature rate performance tests conducted using CR2032 coin cells within the voltage range of 1.1~4.0V revealed that the double-shell FeF3 / carbon cathode material exhibited excellent rate performance at 0℃, 25℃, and 40℃. Even at a low temperature of 0℃ and a high rate of 5C, it maintained a discharge capacity of approximately 250mAh / g. In contrast, Comparative Example 1, tested with CR2032 coin cells under the same conditions, failed to recharge after the first discharge at 0℃. These results indicate that when the double-shell FeF3 / carbon cathode material is used in lithium-ion batteries, it demonstrates ideal capacity density, excellent cycle stability, and capacity retention compared to products without double-shell coating modification under wide temperature range and high rate cycling conditions.

[0099] Therefore, it can be seen that the synergistic effect of the double-shell structure provides dual fixation and buffering for the matrix material, namely the conversion cathode material, enabling the double-shell structure conversion cathode material to exhibit ideal capacity density, rate performance, excellent cycle stability and cycle life, and is suitable for high-rate cycling conditions in a wide temperature range.

[0100] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A double-shell structure conversion cathode material, characterized in that, It includes a substrate material and an inner shell layer and an outer shell layer sequentially covering the surface of the substrate material, wherein the substrate material is a conversion-type positive electrode material, the inner shell layer is a liquid metal, and the outer shell layer is a metal oxide.

2. The double-shell structure conversion cathode material according to claim 1, characterized in that, The mass ratio of the conversion-type cathode material to the liquid metal is 50:(1~10).

3. The double-shell structure conversion cathode material according to claim 1, characterized in that, The mass ratio of the conversion-type cathode material to the metal oxide is 50:(1~5); and / or The thickness of the outer shell layer is 2nm~7nm.

4. The double-shell structure conversion cathode material according to claim 1, characterized in that, The liquid metal includes at least one of gallium, gallium-tin alloy, gallium-indium alloy, and gallium-indium-tin alloy; and / or The metal oxide includes at least one of tin oxide, zinc oxide, titanium dioxide, silicon dioxide, zirconium oxide, aluminum oxide, cobalt tetroxide, tungsten oxide, and bismuth oxide.

5. The double-shell structure conversion cathode material according to claim 1, characterized in that, The conversion-type cathode material includes at least one of elemental sulfur, sulfides, transition metal halides, and transition metal halide / carbon composite materials.

6. A method for preparing a double-shell structure conversion cathode material, characterized in that, include: A mixture is obtained by mixing a matrix material, liquid metal, and a dispersion medium, wherein the matrix material is a conversion-type cathode material; The mixture is physically dispersed at a temperature above the melting point of the liquid metal, causing the liquid metal to coat the surface of the matrix material to form an inner shell, thereby obtaining a cathode material intermediate; and The cathode material intermediate is dispersed in a liquid medium containing metal elements, and a layer of metal oxide is synthesized in situ on the surface of the inner shell layer using a wet chemical synthesis method to form an outer shell layer, thereby obtaining the double-shell structure conversion cathode material.

7. The method for preparing the double-shell structure conversion cathode material according to claim 6, characterized in that, The liquid metal includes at least one of gallium, gallium-tin alloy, gallium-indium alloy, and gallium-indium-tin alloy; and / or The mass ratio of the conversion-type cathode material to the liquid metal is 50:(1~10).

8. The method for preparing the double-shell structure conversion cathode material according to claim 6, characterized in that, The physical dispersion includes at least one of planetary ball milling, high-energy ball milling, ultrasonic dispersion, high-speed shear dispersion, high-speed grinding, mechanical stirring dispersion, and high-pressure homogenization dispersion; and / or The wet chemical synthesis method includes at least one of the sol-gel method, hydrothermal method, and in-situ alcoholysis method.

9. The method for preparing the double-shell structure conversion cathode material according to claim 8, characterized in that, The wet chemical synthesis method is an in-situ alcoholysis method. The reaction conditions for the in-situ alcoholysis method include: a reaction temperature of 30℃~60℃, a reaction time of 24h~48h, and the liquid medium includes organometallic salts and alcohols.

10. An electrochemical device, characterized in that, The invention includes a positive electrode sheet, wherein the positive electrode sheet includes a positive electrode material, and the positive electrode material is a double-shell structure conversion type positive electrode material as described in any one of claims 1 to 5 or a double-shell structure conversion type positive electrode material prepared by the preparation method of the double-shell structure conversion type positive electrode material as described in any one of claims 6 to 9.

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