Composite positive electrode material, preparation method thereof and battery

Abstract

The invention relates to a composite positive electrode material which is of a core-shell structure and comprises an inner core, a first coating layer and a second coating layer, and the inner core is sequentially coated with the first coating layer and the second coating layer from inside to outside. The first coating layer comprises barium selenide, and the second coating layer comprises carbon quantum dots. In the application, double-layer coating of the first coating layer and the second coating layer is constructed on the surface of the inner core, and the barium selenide and the carbon quantum dots can synergistically form a perfect and reliable conductive network, so that the overall electronic conductivity of the composite positive electrode material can be remarkably improved, the charge transfer impedance can be reduced, and the migration of active ions can be promoted; and thus, the high-rate performance and the cycle performance of the composite positive electrode material can be improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a composite cathode material, its preparation method, and a battery. Background Technology

[0002] Lithium iron phosphate (LFP) cathode materials possess advantages such as high safety, long cycle life, environmental friendliness, and low cost, and are currently widely used in power batteries and energy storage batteries. However, this cathode material itself has inherent defects such as low intrinsic electronic conductivity and slow lithium-ion diffusion rate, which significantly limits its rate performance and cycle performance.

[0003] To improve the conductivity and ion transport kinetics of lithium iron phosphate (LFP) batteries, the industry commonly employs modification techniques such as carbon coating, ion doping, and particle nanostructuring. Among these, carbon coating is one of the most widely used technologies. By coating the surface of LFP particles with a carbon layer, the overall electronic conductivity of the material can be improved, thereby enhancing the rate performance and cycle life of LFP cathode materials.

[0004] However, the carbon coating layers obtained by pyrolysis of traditional carbon sources (such as glucose and sucrose) are usually amorphous carbon with limited conductivity, and the coating is prone to unevenness and discontinuity, resulting in an imperfect electron conduction network. Therefore, the improvement on the rate performance and cycle performance of lithium iron phosphate cathode materials is relatively limited. Summary of the Invention

[0005] In view of this, the present application provides a composite cathode material, a method for preparing the same, and a battery to solve at least one problem existing in the prior art.

[0006] In a first aspect, embodiments of this application provide a composite cathode material, which has a core-shell structure, including a core and a first coating layer and a second coating layer that sequentially cover the core from the inside out. The first coating layer comprises barium selenide, and the second coating layer comprises carbon quantum dots.

[0007] In conjunction with the first aspect of this application, in an optional embodiment, the core material comprises lithium iron phosphate, the surface of which is coated with a carbon coating layer; and / or, the core has a particle size of 200 nm to 1.5 μm.

[0008] In conjunction with the first aspect of this application, in an optional embodiment, the thickness of the first coating layer is 1 nm to 15 nm; and / or, the thickness of the second coating layer is 1 nm to 15 nm; and / or, the mass ratio of the first coating layer to the second coating layer is (1:2) to (1:1).

[0009] Secondly, embodiments of this application provide a method for preparing a composite cathode material, the method comprising the following steps: S1: Mix the core material, barium source and selenium source, add solvent, ball mill, and dry to obtain the first mixed powder; S2: Under a first inert gas atmosphere, the first mixed powder is subjected to a first heat treatment to form a first coating layer on the surface of the core, the first coating layer comprising barium selenide; S3: Add the core with the first coating layer on its surface to the carbon quantum dot dispersion, and after dispersion treatment, obtain a mixed slurry; dry the mixed slurry to obtain a second mixed powder; S4: Under a second inert gas atmosphere, the second mixed powder is subjected to a second heat treatment to form a second coating layer on the surface of the first coating layer. The second coating layer includes carbon quantum dots to obtain the composite cathode material.

[0010] In conjunction with the second aspect of this application, in an optional embodiment, step S1 satisfies at least one of the following features: (1) The core material includes lithium iron phosphate, and the surface of the lithium iron phosphate is coated with a carbon coating layer; (2) The particle size of the core is 200 nm to 1.5 μm; (3) The barium source includes barium acetate and / or barium carbonate; (4) The selenium source includes selenium powder and / or benzene-selenophenol; (5) The solvent includes anhydrous ethanol; (6) The rotation speed of the ball mill is 300 r / min to 500 r / min, and the time is 4 h to 6 h; (7) The drying process is performed at a temperature of 80℃~100℃ for 10h~14h.

[0011] In conjunction with the second aspect of this application, in an optional embodiment, in step S2, the temperature of the first heat treatment is 500℃~700℃, and the holding time is 3h~5h; and / or, the thickness of the first coating layer is 1nm~15nm.

[0012] In conjunction with the second aspect of this application, in an optional embodiment, step S3, the dispersion treatment includes: first performing a stirring treatment, and then performing an ultrasonic treatment; optionally, the stirring treatment time is 5h~6h, and the ultrasonic treatment time is 30min~60min.

[0013] In conjunction with the second aspect of this application, in an optional embodiment, in step S4, the temperature of the second heat treatment is 350°C to 400°C, and the holding time is 2h to 4h; and / or, the thickness of the second coating layer is 1nm to 15nm; and / or, the mass ratio of the first coating layer to the second coating layer is (1:2) to (1:1).

[0014] In conjunction with a second aspect of this application, in an optional embodiment, the preparation steps of the carbon quantum dot dispersion include: Carbon quantum dots are obtained by heating a carbon source, followed by melting and carbonization. The carbon quantum dot solid was dissolved in deionized water and mixed evenly to obtain the carbon quantum dot dispersion. The preparation steps of the carbon quantum dot dispersion satisfy at least one of the following characteristics: (1) The carbon source includes at least one of citric acid, chitosan, vitamin C, and amino acids; (2) The temperature of the heat treatment is 180℃~220℃; (3) The mixing process is ultrasonic treatment.

[0015] Thirdly, embodiments of this application provide a battery including a positive electrode sheet, wherein the positive electrode sheet includes the composite positive electrode material described in any one of the first aspects or the composite positive electrode material prepared by the method described in any one of the second aspects.

[0016] Compared with the prior art, the embodiments of this application have the following beneficial effects: The composite cathode material, its preparation method, and battery provided in this application embodiment are core-shell structures, including a core and a first coating layer and a second coating layer that sequentially cover the core from the inside out; the first coating layer includes barium selenide, and the second coating layer includes carbon quantum dots. Barium selenide is a semiconductor material with high electronic conductivity. Furthermore, the large size of barium ions in barium selenide prevents deep doping into the core during high-temperature coating processes, thus avoiding impact on the core lattice structure. In other words, the first coating layer effectively modifies the interface, significantly reducing the charge transfer impedance on the surface of the core cathode active material particles and promoting the migration of active ions. Additionally, the relatively stable valence state of barium ions in barium selenide prevents them from becoming centers of interfacial side reactions, ensuring the durability of the first coating layer during battery cycling. Carbon quantum dots are zero-dimensional nanomaterials with extremely small sizes, capable of uniformly coating the first coating layer and synergistically forming efficient electron transport channels. In this embodiment, by constructing a double-layer coating of first and second coating layers on the core surface, barium selenide and carbon quantum dots synergistically form a complete and reliable conductive network, thereby significantly improving the overall electronic conductivity of the composite cathode material, reducing charge transfer impedance, promoting the migration of active ions, and ultimately enhancing the high-rate performance and cycle performance of the composite cathode material.

[0017] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0018] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a schematic flowchart illustrating a method for preparing a composite cathode material according to an embodiment of this application. Detailed Implementation

[0019] To make the technical solution and beneficial effects of the present invention more apparent and understandable, a detailed description is provided below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments, unless otherwise specified, are generally performed under conventional experimental conditions. Unless otherwise specified, all reagents and raw materials used in this invention are commercially available.

[0020] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of this application. However, it will be apparent to those skilled in the art that this application can be practiced without one or more of these details. In other instances, to avoid confusion with this application, some technical features well-known in the art have not been described; that is, not all features of actual embodiments are described herein, nor are well-known functions and steps described in detail.

[0021] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this 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. It should also be understood that the terms “comprising” and / or “including,” when used in this specification, identify the presence of the stated 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 associated listed items.

[0022] To fully understand this application, detailed steps and structures will be presented in the following description to illustrate the technical solution of this application. Preferred embodiments of this application are described in detail below; however, in addition to these detailed descriptions, this application may have other implementation methods.

[0023] Unless otherwise defined, the technical and scientific terms used in this application have the same meanings as those in the technical and scientific field to which this application pertains.

[0024] Unless otherwise specified, the techniques or conditions described in the following embodiments are generally performed in accordance with conventional techniques or conditions described in the literature in this field, or in accordance with the product manual and the manufacturer's recommendations. All numerical ranges in the following embodiments include endpoint values.

[0025] Selenides possess excellent electrical conductivity and have been introduced as a novel coating material for modifying lithium iron phosphate materials. However, some selenides contain alkali metal ions, such as Na+. + K + These ions, with their small ionic radii and high mobility, readily become deeply doped into the lithium iron phosphate lattice during high-temperature selenide coating, leading to uncontrollable distortions in the main lithium iron phosphate structure and potentially blocking lithium-ion transport channels. Furthermore, some metal selenides contain transition metals, which are highly reactive and easily dissolve, becoming centers of side reactions and causing instability in the selenide coating layer. Moreover, current methods for improving the rate performance and cycle performance of lithium iron phosphate through single selenide coating are not yet ideal.

[0026] Based on this, embodiments of this application provide a composite cathode material, which has a core-shell structure, including a core and a first coating layer and a second coating layer that sequentially coat the core from the inside out; the first coating layer includes barium selenide, and the second coating layer includes carbon quantum dots.

[0027] Barium selenide (BaSe) is a semiconductor material with high electronic conductivity. The large size of barium ions in barium selenide prevents deep doping into the core during high-temperature coating processes, thus avoiding impact on the core lattice structure. In other words, the first coating layer effectively modifies the interface, significantly reducing the charge transfer impedance on the surface of the core cathode active material particles and promoting the migration of active ions (such as lithium or sodium ions). Furthermore, the relatively stable valence state of barium ions in barium selenide prevents them from becoming centers of interfacial side reactions, ensuring the durability of the first coating layer during battery cycling. Carbon quantum dots are zero-dimensional nanomaterials with extremely small sizes, capable of uniformly coating the first coating layer and synergistically forming efficient electron transport channels. In this embodiment, by constructing a double-layer coating of first and second coating layers on the core surface, barium selenide and carbon quantum dots synergistically form a complete and reliable conductive network, thereby significantly improving the overall electronic conductivity of the composite cathode material, reducing charge transfer impedance, promoting the migration of active ions, and ultimately enhancing the high-rate performance and cycle performance of the composite cathode material.

[0028] It should be noted that, in the embodiments of this application, the first coating layer can cover the entire surface of the core. Specifically, it can be understood that, within the allowable deviations of the process in this field, the first coating layer covers the entire surface of the core. Similarly, within the allowable deviations of the process in this field, the second coating layer can cover the entire surface of the first coating layer.

[0029] In some specific embodiments, the first coating layer can be barium selenide, and the second coating layer can be carbon quantum dots.

[0030] In this embodiment, the core is a positive electrode active material. The core material can be a commonly used positive electrode active material in the art, specifically, at least one of lithium iron phosphate, ternary materials, and lithium manganese iron phosphate.

[0031] In some embodiments, the core material may include lithium iron phosphate, the surface of which is coated with a carbon coating.

[0032] Lithium iron phosphate (LFP) has low intrinsic electronic conductivity. Coating the surface of LFP with a carbon coating layer can improve the conductivity of the core and enhance the coating effect of the first and second coating layers. Through the synergistic effect of the carbon coating layer, the first coating layer, and the second coating layer, the electronic conductivity of LFP can be significantly improved, promoting lithium-ion diffusion. This allows LFP cathode materials to maintain advantages such as high safety, long cycle life, environmental friendliness, and low cost while also achieving high rate performance and high cycle performance.

[0033] When the core particle size is too small, it leads to a decrease in the compaction density of the composite cathode material and a deterioration in processing performance; when the core particle size is too large, it leads to an excessively long diffusion path for lithium ions, thereby affecting the rate performance of the composite cathode material. Therefore, in some embodiments, the core particle size can be 200 nm to 1.5 μm, for example, 200 nm, 400 nm, 600 nm, 800 nm, 1.0 μm, 1.2 μm, 1.4 μm, 1.5 μm, or any value between any two of the above ranges.

[0034] When the mass ratio of the first coating layer to the second coating layer is too small, the proportion of the first coating layer is too small and the proportion of the second coating layer is too large. Excessive carbon quantum dots in the second coating layer can easily clog the gaps between the active electrode particles, increasing the resistance to lithium-ion transport. When the mass ratio of the first coating layer to the second coating layer is too large, the proportion of the first coating layer is too large and the proportion of the second coating layer is too small. This is not conducive to the complete and uniform coating of the first coating layer by carbon quantum dots, thereby reducing the integrity of the overall electronic network constructed by the first and second coating layers. Therefore, in some embodiments, the mass ratio of the first coating layer to the second coating layer can be (1:2) to (1:1), for example, 1:2, 1:1.5, 1:1, or any value within any two of the above ranges.

[0035] In some embodiments, the thickness of the first coating layer can be 1 nm to 15 nm, for example, it can be 1 nm, 3 nm, 5 nm, 7 nm, 9 nm, 11 nm, 13 nm, 15 nm, or any value between any two of the above ranges. In this way, while ensuring that the first coating layer can fully play its role, the lithium ion transport path can be avoided from being excessively prolonged.

[0036] In some embodiments, the thickness of the second coating layer can be 1 nm to 15 nm, for example, 1 nm, 3 nm, 5 nm, 7 nm, 9 nm, 11 nm, 13 nm, 15 nm, or any value between any two of the above ranges. This ensures that the second coating layer can fully function while avoiding excessively extending the lithium-ion transport path.

[0037] This application also provides a method for preparing a composite cathode material; please refer to [reference needed].Figure 1 The method for preparing the composite cathode material provided in this application includes the following steps: S1: Mix the core material, barium source and selenium source, add solvent, ball mill, and dry to obtain the first mixed powder; S2: Under a first inert gas atmosphere, the first mixed powder is subjected to a first heat treatment to form a first coating layer on the surface of the core, the first coating layer comprising barium selenide; S3: The core with the first coating layer on its surface is added to the carbon quantum dot dispersion, and after dispersion treatment, a mixed slurry is obtained; the mixed slurry is dried to obtain a second mixed powder; S4: Under a second inert gas atmosphere, the second mixed powder is subjected to a second heat treatment to form a second coating layer on the surface of the first coating layer. The second coating layer includes carbon quantum dots to obtain a composite cathode material.

[0038] In this embodiment, by combining liquid phase mixing and solid phase sintering, a first coating layer and a second coating layer are sequentially coated on the surface of the core. The process is simple and easy to implement, the cost is controllable, the equipment requirements are low, and it is easy to achieve large-scale industrial production. The first coating layer comprises barium selenide, and the second coating layer comprises carbon quantum dots. Barium selenide is a semiconductor material with high electronic conductivity, and the barium ions in barium selenide have a large size, which can avoid deep doping of only the core in the high-temperature coating process and affecting the core lattice structure. That is, the first coating layer can play a better role in interface modification, which can greatly reduce the charge transfer impedance on the surface of the core cathode active material particles and promote the migration of active ions. The barium ions in barium selenide have a relatively stable valence state and can also avoid becoming the center of interface side reactions, ensuring the durability of the first coating layer in battery cycling. Carbon quantum dots are zero-dimensional nanomaterials with extremely small size, which can be uniformly coated on the first coating layer to form an efficient electron transport channel. In the embodiments of this application, by constructing a double coating of the first coating layer and the second coating layer on the core surface, barium selenide and carbon quantum dots can synergistically form a complete and reliable conductive network, thereby significantly improving the overall electronic conductivity of the composite cathode material, reducing the charge transfer impedance, promoting the migration of active ions, and thus improving the high-rate performance and cycle performance of the composite cathode material.

[0039] In step S1, the core material, barium source, and selenium source are mixed, a solvent is added, and the mixture is ball-milled and dried to obtain the first mixed powder.

[0040] In the actual preparation process, the barium source and selenium source can be weighed according to the molar ratio of barium in the barium source to selenium in the selenium source being 1:1. The core material, barium source and selenium source are put into a ball mill jar, anhydrous ethanol is added, and the mixture is ball-milled to make the materials fully mixed. Then, the ball-milled mixture is placed in a vacuum drying oven for thorough drying.

[0041] For example, the barium source may include barium acetate and / or barium carbonate. Specifically, the barium source may be barium acetate and / or barium carbonate.

[0042] For example, the selenium source may include selenium powder and / or selenophenol. Specifically, the selenium source may be selenium powder and / or selenophenol.

[0043] For example, the solvent may include anhydrous ethanol. Specifically, the solvent may be anhydrous ethanol.

[0044] In some embodiments, the rotational speed of the ball mill can be 300 r / min to 500 r / min, for example, 300 r / min, 350 r / min, 400 r / min, 450 r / min, 500 r / min or any value between any two of the above ranges, and the ball milling time can be 4 h to 6 h, for example, 4 h, 4.5 h, 5 h, 5.5 h, 6 h or any value between any two of the above ranges.

[0045] In some embodiments, the drying temperature can be 80°C to 100°C, for example, 80°C, 85°C, 90°C, 95°C, 100°C or any value between any two of the above ranges, and the time can be 10h to 14h, for example, 10h, 11h, 12h, 13h, 14h or any value between any two of the above ranges.

[0046] In some embodiments, the particle size of the core can be 200 nm to 1.5 μm, for example, 200 nm, 400 nm, 600 nm, 800 nm, 1.0 μm, 1.2 μm, 1.4 μm, 1.5 μm, or any value between any two of the above ranges. This is beneficial for balancing the compaction density, processing performance, and rate performance of the final composite cathode material.

[0047] In this embodiment, the core is a positive electrode active material. The core material can be a commonly used positive electrode active material in the art, specifically, at least one of lithium iron phosphate, ternary materials, and lithium manganese iron phosphate.

[0048] In some embodiments, the core material may include lithium iron phosphate, the surface of which is coated with a carbon coating.

[0049] Lithium iron phosphate (LFP) has low intrinsic electronic conductivity. Coating the surface of LFP with a carbon coating layer can improve the conductivity of the core and enhance the coating effect of the first and second coating layers. Through the synergistic effect of the carbon coating layer, the first coating layer, and the second coating layer, the electronic conductivity of LFP can be significantly improved, promoting lithium-ion diffusion. This allows LFP cathode materials to maintain advantages such as high safety, long cycle life, environmental friendliness, and low cost while also achieving high rate performance and high cycle performance.

[0050] In some embodiments, the method for preparing lithium iron phosphate may include: Step S11: Mix the lithium source, iron source, phosphorus source and the first carbon source, perform wet ball milling, and after drying, obtain precursor powder.

[0051] Here, the lithium source can be, for example, Li₂CO₃; the iron source can be, for example, FeC₂O₄·2H₂O; the phosphorus source can be, for example, NH₄H₂PO₄; and the first carbon source can be, for example, at least one of glucose, sucrose, and citric acid. The molar ratio of lithium in the lithium source, iron in the iron source, and phosphorus in the phosphorus source can be, for example, 1.05:1:1. The mass of the first carbon source can be, for example, 13% to 15% of the mass of the iron source. This is beneficial for improving the quality of the lithium iron phosphate obtained by subsequent sintering and the integrity of the carbon coating layer on the lithium iron phosphate.

[0052] In the actual preparation process, lithium source, iron source, phosphorus source and first carbon source can be put into a ball mill together, ethanol is added, and the mixture is ball-milled at a speed of 300 r / min to 500 r / min for 6 h to 12 h; then the ball-milled mixture is placed in a vacuum drying oven and dried thoroughly at 80 ℃ to 100 ℃ to obtain dry precursor powder.

[0053] Step S12: Under inert gas protection, the precursor powder is sintered at a temperature of 500℃~700℃ to obtain lithium iron phosphate with a carbon coating layer on the surface.

[0054] In the actual preparation process, the dried precursor powder can be transferred to an alumina crucible, then placed in a tube furnace, heated to 500℃~700℃ under an argon atmosphere, and held for 10h~14h to crystallize and carbon-coat the product, thus obtaining carbon-coated lithium iron phosphate (which can be denoted as LFP / C).

[0055] In step S2, the first mixed powder is subjected to a first heat treatment under a first inert gas atmosphere to form a first coating layer on the surface of the core. The first coating layer includes barium selenide.

[0056] In the actual preparation process, the first mixed powder can be first transferred to an alumina crucible, then placed in a tube furnace, and heated to the temperature of the first heat treatment under a first inert gas atmosphere. This allows for the coating of the core with barium selenide, i.e., the first coating layer is barium selenide. Exemplarily, the first inert gas can be at least one of argon, nitrogen, and helium.

[0057] In some embodiments, the temperature of the first heat treatment can be 500℃~700℃, for example, 500℃, 550℃, 600℃, 650℃, 700℃ or any value between any two of the above ranges, and the holding time can be 3h~5h, for example, 3h, 3.5h, 4h, 4.5h, 5h or any value between any two of the above ranges. This facilitates the full reaction of the barium source and selenium source at high temperature, so as to form a relatively complete and uniform first coating layer on the surface of the core, thereby improving the coating effect and quality of the first coating layer.

[0058] In some embodiments, the thickness of the first coating layer formed in step S2 can be 1 nm to 15 nm, for example, it can be 1 nm, 3 nm, 5 nm, 7 nm, 9 nm, 11 nm, 13 nm, 15 nm, or any value between any two of the above ranges. In this way, while ensuring that the first coating layer can fully play its role, the excessive extension of the lithium ion transport path can be avoided.

[0059] In step S3, the core with the first coating layer on its surface is added to the carbon quantum dot dispersion and dispersed to obtain a mixed slurry; the mixed slurry is dried to obtain a second mixed powder.

[0060] In some embodiments, the preparation steps of the carbon quantum dot dispersion may include: Step S31: Heat the carbon source, melt and carbonize it to obtain solid carbon quantum dots.

[0061] In the actual preparation process, the carbon source can be placed in a beaker and heated in a muffle furnace. The carbon source will melt and carbonize, generating brownish-yellow carbon quantum dot solids. The heating temperature can be between 180℃ and 220℃, for example, 180℃, 190℃, 200℃, 210℃, 220℃, or any value between any two of the above ranges.

[0062] The carbon source in the embodiments of this application can also be regarded as a second carbon source. Exemplarily, the carbon source may include at least one of citric acid, chitosan, vitamin C, and amino acids. Specifically, the carbon source may be at least one of citric acid, chitosan, vitamin C, and amino acids.

[0063] Step S32: Dissolve solid carbon quantum dots in deionized water and mix thoroughly to obtain a carbon quantum dot dispersion.

[0064] In actual preparation processes, the mixing process can be ultrasonic treatment. Ultrasonic treatment allows carbon quantum dots to be more uniformly dispersed in deionized water.

[0065] In some embodiments, step S3, the dispersion treatment may include: first performing a stirring treatment, and then performing an ultrasonic treatment. Optionally, the stirring treatment time may be 5 h to 6 h, and the ultrasonic treatment time may be 30 min to 60 min. This helps to ensure that the core of the first coating layer is fully and uniformly dispersed in the carbon quantum dot dispersion, so that the carbon quantum dots can be adsorbed relatively uniformly and completely on the surface of the first coating layer, thereby improving the uniformity and integrity of the subsequently formed second coating layer and ensuring the quality of the second coating layer.

[0066] In the actual preparation process, the core with the first coating layer obtained in step S2 above can be slowly added to the carbon quantum dot dispersion under magnetic stirring. After the addition is complete, stirring is continued for 5 to 6 hours, followed by ultrasonic treatment. Finally, the uniformly mixed slurry is dried under stirring at 80°C to obtain the second mixed powder.

[0067] In step S4, the second mixed powder is subjected to a second heat treatment under a second inert gas atmosphere to form a second coating layer on the surface of the first coating layer. The second coating layer includes carbon quantum dots, thus obtaining a composite cathode material.

[0068] In the actual preparation process, the second mixed powder can be placed in a tube furnace, heated to the temperature of the second heat treatment under a second inert gas atmosphere, and held at that temperature for a preset time. Carbon quantum dots are then coated onto the surface of the first coating layer, i.e., the second coating layer is carbon quantum dots. After cooling, a composite cathode material is obtained, i.e., a double-coated composite cathode material. For example, the second inert gas can be at least one of argon, nitrogen, and helium.

[0069] In some embodiments, the temperature of the second heat treatment can be 350°C to 400°C, for example, 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, or any value between any two of the above ranges. The holding time can be 2 hours to 4 hours, for example, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, or any value between any two of the above ranges. This is beneficial for improving the bonding force between the carbon quantum dots and the first coating layer, promoting the formation of a more complete and uniform second coating layer on the surface of the first coating layer, thereby improving the coating effect and quality of the second coating layer; at the same time, it avoids excessively high temperatures that could damage the carbon quantum dot structure.

[0070] In some embodiments, the thickness of the second coating layer formed in step S4 can be 1 nm to 15 nm, for example, it can be 1 nm, 3 nm, 5 nm, 7 nm, 9 nm, 11 nm, 13 nm, 15 nm, or any value between any two of the above ranges. In this way, while ensuring that the second coating layer can fully play its role, the lithium ion transport path can be avoided from being excessively prolonged.

[0071] In some embodiments, the mass ratio of the first coating layer to the second coating layer can be (1:2) to (1:1), for example, 1:2, 1:1.5, 1:1, or any value within any two of the above ranges. This facilitates the construction of a more complete conductive network through the first and second coating layers, thereby further improving the overall electronic conductivity of the composite cathode material, better promoting the transport of electrons and active ions, and ultimately enhancing the high-rate performance and cycle performance of the composite cathode material.

[0072] This application also provides a battery, including a positive electrode sheet, which includes the composite positive electrode material described in any of the foregoing embodiments or the composite positive electrode material prepared by the method described in any of the foregoing embodiments.

[0073] It is understood that the beneficial effects of the composite cathode material described in any of the foregoing embodiments, or the composite cathode material prepared by the method including the composite cathode material described in any of the foregoing embodiments, are applicable to the battery in the embodiments of this application.

[0074] Specifically, the positive electrode sheet may include a positive current collector and a positive active material layer located on at least one surface of the positive current collector along the thickness direction. The positive active material layer includes the composite positive electrode material described in any of the foregoing embodiments or a composite positive electrode material prepared by the preparation method of the composite positive electrode material described in any of the foregoing embodiments.

[0075] In some embodiments, the battery can be a lithium-ion battery. Typically, the battery also includes a negative electrode, an electrolyte, and a separator. During charging and discharging, lithium ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor for lithium ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing lithium ions to pass through.

[0076] In some embodiments, the battery preparation method may include: winding or stacking a negative electrode sheet, a separator, and a positive electrode sheet to obtain a single bare cell; assembling the single bare cell into a casing (specifically, for example, an aluminum-plastic film, an aluminum-steel casing, etc.); and then drying, injecting electrolyte, encapsulating, forming, and capacity testing to obtain the battery. The separator may be selected from at least one of polyolefin separators, ceramic-coated separators, and cellulose separators; the electrolyte may be selected from at least one of carbonate electrolytes, ether electrolytes, and ionic liquid electrolytes; and the negative electrode active material in the negative electrode sheet may be selected from at least one of graphite, silicon-based materials, and lithium metal.

[0077] The technical solution of this application will be further described below with reference to several embodiments and comparative examples.

[0078] Example 1 The preparation method of the composite cathode material in this embodiment includes the following steps: S101: Weigh 7.76g Li2CO3 (lithium source), 36g FeC2O4·2H2O (iron source), and 23g NH4H2PO4 (phosphorus source) according to the molar ratio of n(Li):n(Fe):n(P) = 1.05:1:1, and then weigh 6.70g glucose (first carbon source); put all the above-weighed raw materials into a ball mill, add ethanol, and ball mill at 400 r / min for 12 h; place the ball-milled mixture in a vacuum drying oven and dry it thoroughly at 100℃ for 12 h to obtain dry precursor powder; transfer the dried precursor powder to an alumina crucible, and then place it in a tube furnace, and heat it to 600℃ at a heating rate of 5℃ / min under an argon atmosphere, and hold it at that temperature for 12 h for crystallization and carbon coating to obtain carbon-coated lithium iron phosphate (LFP / C), which is used as the core material; S102: Weigh 10g of the core material obtained in step S101, and weigh 0.68g of barium acetate and 0.21g of selenium powder according to the molar ratio of n(Ba):n(Se)=1:1. Put all three into a ball mill jar, add a small amount of anhydrous ethanol, and ball mill at 400r / min for 6h to ensure that the materials are fully mixed. After the mixture is finished, place it in a vacuum drying oven and dry it thoroughly at 100℃ for 12h to obtain the first mixed powder. S103: The first mixed powder obtained in step S102 is transferred to an alumina crucible and then placed in a tube furnace. Under an argon atmosphere, it is heated to 600°C at a heating rate of 5°C / min and held for 5 hours (first heat treatment). Barium selenide is coated on the surface of the core to form a first coating layer, resulting in barium selenide coated lithium iron phosphate (BaSe@LFP / C). The thickness of the first coating layer is 3nm. S104: Weigh 2g of citric acid (carbon source) into a beaker and heat it in a muffle furnace at 200℃ for 30min. The citric acid will melt and carbonize to form brownish-yellow carbon quantum dot solids. Dissolve the obtained carbon quantum dot solids in 100ml of deionized water and sonicate for 1.5h to obtain a carbon quantum dot dispersion. S105: Measure 50 mL of the carbon quantum dot dispersion prepared in step S104 and add it to a beaker. Weigh 5 g of BaSe@LFP / C powder prepared in step S103 and slowly add the powder to the carbon quantum dot dispersion under magnetic stirring. After the addition is complete, continue stirring for 6 h and sonicate for 60 min. Dry the mixed slurry under stirring at 80 °C to obtain the second mixed powder. S106: The second mixed powder is placed in a tube furnace and heated to 400°C at a heating rate of 5°C / min under an argon atmosphere and held for 4 hours (second heat treatment). Carbon quantum dots are coated on the surface of the first coating layer to form a second coating layer. After cooling, the final CQDs / BaSe@LFP / C, i.e., double-coated lithium iron phosphate composite material, is obtained. The thickness of the second coating layer is 2nm.

[0079] Example 2 The preparation of the composite cathode material in this embodiment is basically the same as in Example 1, except that: In step S103, the temperature of the first heat treatment is adjusted from 600℃ to 500℃; the thickness of the first coating layer is 1nm.

[0080] Example 3 The preparation of the composite cathode material in this embodiment is basically the same as in Example 1, except that: In step S103, the temperature of the first heat treatment is adjusted from 600℃ to 700℃; the thickness of the first coating layer is 5nm.

[0081] Comparative Example 1 In this comparative example, the preparation steps of the composite cathode material are the same as step S101 in Example 1, that is, steps S102-S106 are omitted compared to Example 1; the obtained composite cathode material is carbon-coated lithium iron phosphate (LFP / C).

[0082] Comparative Example 2 In this comparative example, the preparation steps of the composite cathode material are the same as steps S101-S103 in Example 1, that is, steps S104-S106 are omitted compared to Example 1; the obtained composite cathode material is barium selenide-coated lithium iron phosphate (BaSe@LFP / C).

[0083] The composite cathode materials prepared in the above embodiments and comparative examples were applied in batteries, and the electrochemical performance of the batteries was tested.

[0084] The battery preparation method includes the following steps: the composite positive electrode material, binder polyvinylidene fluoride, and conductive agent SP prepared in the above embodiments and comparative examples are dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 80:10:10 and stirred to form a uniform and stable slurry. The slurry is then coated on aluminum foil, dried to obtain a positive electrode sheet, which is then cut, weighed, and placed in a glove box. In the glove box, a lithium metal sheet is used as the negative electrode, a solution of LiPF6 dissolved in diethylene glycol dimethyl ether is used as the electrolyte, and a polypropylene membrane is used as the separator. The CR2032 button battery is assembled in an argon-filled glove box.

[0085] The electrochemical performance of the button cells prepared above was tested using a battery testing system within a voltage window of 2.5V to 3.65V. The specific tests are as follows: (1) 5C discharge capacity retention test: The battery was left to stand at 25℃ for 1 hour, discharged at a constant current of 0.33C to 2.5V, left to stand for 30 minutes, charged at a constant current of 0.33C to 3.65V, charged at a constant voltage of 3.65V until the current is less than or equal to 0.05C, left to stand for 30 minutes, and discharged at a constant current of 0.33C to 2.5V. The discharge capacity C1 was recorded at this time; left to stand for 30 minutes, charged at a constant current of 0.33C to 3.65V, charged at a constant voltage of 3.65V until the current is less than or equal to 0.05C, left to stand for 30 minutes, and discharged at a constant current of 5C to 2.5V. The discharge capacity C2 was recorded at this time; 5C discharge capacity retention rate = (C2 / C1) × 100%; (2) Capacity retention test after 500 cycles: The battery was left to stand at 25°C for 1 hour, discharged at a constant current of 0.33C to 2.5V, left to stand for 30 minutes, charged at a constant current of 0.33C to 3.65V, charged at a constant voltage of 3.65V until the current was less than or equal to 0.05C, left to stand for 30 minutes, and discharged at a constant current of 0.33C to 2.5V. The discharge capacity at this time was recorded as the discharge capacity of the first cycle. After standing for 30 minutes, the capacity of the lithium-ion battery at the 500th cycle was recorded as the discharge capacity of the 500th cycle. The capacity retention rate after 500 cycles = (discharge capacity of the 500th cycle / discharge capacity of the 1st cycle) × 100%; (3) Electrochemical impedance spectroscopy (EIS) test: An electrochemical workstation was used, the test frequency range was set to 100kHz~10mHz, the amplitude was set to 5mV, and the test temperature was 25℃; the data analysis used ZView software to fit the equivalent circuit to extract the charge transfer impedance (Rct) and solution impedance (R).

[0086] The test results are shown in Table 1.

[0087] Table 1

[0088] As can be seen from the data in Table 1, the composite cathode material prepared in Comparative Example 1 is carbon-coated lithium iron phosphate. The corresponding battery exhibits significantly lower capacity retention at high-rate 5C discharge and low capacity retention after 500 cycles, along with significantly higher charge transfer impedance. This indicates that simply coating the lithium iron phosphate surface with carbon is insufficient to effectively reduce the charge transfer impedance of the cathode material particles, thus hindering the improvement of high-rate performance and cycle life.

[0089] The composite cathode material prepared in Comparative Example 2 was lithium iron phosphate coated with barium selenide, which is based on Comparative Example 1, with the addition of a barium selenide coating layer on the surface of the carbon coating layer. As can be seen from the data in Table 1, compared to Comparative Example 1, the charge transfer impedance of the battery in Comparative Example 2 is slightly lower, and the 5C discharge capacity retention rate and the capacity retention rate after 500 cycles are slightly improved. However, they are still significantly lower than those in Examples 1 to 3. This indicates that the barium selenide coating layer can play a certain role in reducing the charge transfer impedance on the surface of the cathode material particles; however, simply adding a barium selenide coating layer is still insufficient to effectively improve the high-rate performance and cycle performance of the cathode material.

[0090] As can be seen from the data in Table 1, compared with Comparative Examples 1 and 2, the batteries containing the composite cathode materials prepared in Examples 1 to 3 exhibit significantly reduced charge transfer impedance, and their 5C discharge capacity retention and 500-cycle capacity retention are both significantly better. This indicates that, in this application, by constructing a double-layer coating of a first coating layer and a second coating layer on the core surface, barium selenide and carbon quantum dots can synergistically form a complete and reliable conductive network, thereby significantly improving the overall electronic conductivity of the composite cathode material, reducing charge transfer impedance, promoting the migration of active ions, and thus improving the high-rate performance and cycle performance of the composite cathode material. This demonstrates the synergistic enhancement effect of the double coating of the first and second coating layers.

[0091] Furthermore, compared to Example 1, Example 2 used a lower selenization reaction temperature, i.e., a lower first heat treatment temperature. This may lead to a slower sublimation rate of selenium powder, incomplete selenization reaction, or agglomeration of the first coating layer, resulting in a first coating layer containing an insulating phase. This may cause a decrease in the overall electronic conductivity of the composite cathode material, a slight increase in charge transfer impedance, and consequently a slight impact on the high-rate performance and cycle performance of the battery. Compared to Example 1, Example 3 used a higher selenization reaction temperature, i.e., a higher first heat treatment temperature. This may lead to over-selenization reaction, agglomeration of the first coating layer, decreased coverage, and the formation of an inert interface layer. The active ion diffusion path becomes longer, which may also slightly affect the high-rate performance and cycle performance of the battery. Therefore, in this application, the temperature of the first heat treatment is preferably 550°C to 650°C, more preferably 600°C; this ensures the formation of a more complete and uniform first coating layer on the surface of the core, improves the coating effect and quality of the first coating layer, and further improves the high-rate performance and cycle performance of the composite cathode material.

[0092] It should be noted that the composite cathode material embodiments, composite cathode material preparation method embodiments, and battery embodiments provided in this application belong to the same concept; the technical features in the technical solutions described in each embodiment can be arbitrarily combined without conflict.

[0093] It should be understood that the above embodiments are exemplary and not intended to encompass all possible implementations. Various modifications and changes can be made to the above embodiments without departing from the scope of this disclosure. Similarly, the various technical features of the above embodiments can be arbitrarily combined to form other embodiments of the present invention that may not be explicitly described. Therefore, the above embodiments only illustrate several implementations of the present invention and do not limit the scope of protection of this patent.

Claims

1. A composite cathode material, characterized in that, The composite cathode material has a core-shell structure, including a core and a first coating layer and a second coating layer that sequentially cover the core from the inside out; The first coating layer comprises barium selenide, and the second coating layer comprises carbon quantum dots.

2. The composite cathode material according to claim 1, characterized in that, The core material includes lithium iron phosphate, the surface of which is coated with a carbon coating layer; and / or, the core particle size is 200 nm to 1.5 μm.

3. The composite cathode material according to claim 1, characterized in that, The thickness of the first coating layer is 1 nm to 15 nm; and / or the thickness of the second coating layer is 1 nm to 15 nm; and / or the mass ratio of the first coating layer to the second coating layer is (1:2) to (1:1).

4. A method for preparing a composite cathode material, characterized in that, The method includes the following steps: S1: Mix the core material, barium source and selenium source, add solvent, ball mill, and dry to obtain the first mixed powder; S2: Under a first inert gas atmosphere, the first mixed powder is subjected to a first heat treatment to form a first coating layer on the surface of the core, the first coating layer comprising barium selenide; S3: Add the core with the first coating layer on its surface to the carbon quantum dot dispersion, and after dispersion treatment, obtain a mixed slurry; dry the mixed slurry to obtain a second mixed powder; S4: Under a second inert gas atmosphere, the second mixed powder is subjected to a second heat treatment to form a second coating layer on the surface of the first coating layer. The second coating layer includes carbon quantum dots to obtain the composite cathode material.

5. The method for preparing the composite cathode material according to claim 4, characterized in that, Step S1 satisfies at least one of the following characteristics: (1) The core material includes lithium iron phosphate, and the surface of the lithium iron phosphate is coated with a carbon coating layer; (2) The particle size of the core is 200 nm to 1.5 μm; (3) The barium source includes barium acetate and / or barium carbonate; (4) The selenium source includes selenium powder and / or benzene-selenophenol; (5) The solvent includes anhydrous ethanol; (6) The rotation speed of the ball mill is 300 r / min to 500 r / min, and the time is 4 h to 6 h; (7) The drying process is performed at a temperature of 80℃~100℃ for 10h~14h.

6. The method for preparing the composite cathode material according to claim 4, characterized in that, In step S2, the temperature of the first heat treatment is 500℃~700℃, and the holding time is 3h~5h; and / or, the thickness of the first coating layer is 1nm~15nm.

7. The method for preparing the composite cathode material according to claim 4, characterized in that, In step S3, the dispersion treatment includes: first performing a stirring treatment, and then performing an ultrasonic treatment; optionally, the stirring treatment time is 5h~6h, and the ultrasonic treatment time is 30min~60min.

8. The method for preparing the composite cathode material according to claim 4, characterized in that, In step S4, the temperature of the second heat treatment is 350℃~400℃ and the holding time is 2h~4h; and / or, the thickness of the second coating layer is 1nm~15nm; and / or, the mass ratio of the first coating layer to the second coating layer is (1:2)~(1:1).

9. The method for preparing the composite cathode material according to any one of claims 4-8, characterized in that, The preparation steps of the carbon quantum dot dispersion include: Carbon quantum dots are obtained by heating a carbon source, followed by melting and carbonization. The carbon quantum dot solid was dissolved in deionized water and mixed evenly to obtain the carbon quantum dot dispersion. The preparation steps of the carbon quantum dot dispersion satisfy at least one of the following characteristics: (1) The carbon source includes at least one of citric acid, chitosan, vitamin C, and amino acids; (2) The temperature of the heat treatment is 180℃~220℃; (3) The mixing process is ultrasonic treatment.

10. A battery, comprising a positive electrode, characterized in that, The positive electrode sheet includes the composite positive electrode material according to any one of claims 1-3 or the composite positive electrode material prepared by the method of preparing the composite positive electrode material according to any one of claims 4-9.

Images