Single-atom doped lithium iron manganese phosphate composite material, preparation method and application thereof

Abstract

This application belongs to the field of battery materials technology, and particularly relates to a single-atom-doped lithium manganese iron phosphate composite material, its preparation method, and its application. The single-atom-doped lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core and an outer shell layer covering the outer surface of the core. The outer shell layer includes carbon material and metal single atoms doped in the carbon material. The carbon material coating effectively improves the electrical conductivity of lithium manganese iron phosphate; the doping of metal single atoms effectively inhibits manganese dissolution from the lithium manganese iron phosphate core, provides suitable sites for lithium nucleation, lowers the lithium-ion migration barrier, and increases the lithium-ion migration rate. By doping the carbon material outer shell layer with metal single atoms, the structural stability, thermal stability, and electrical conductivity of the composite material are effectively improved. This enhances the electrochemical performance of lithium-ion batteries, including rate performance, cycle performance, discharge specific capacity, and low-temperature performance.

Description

Technical Field

[0001] This application belongs to the field of battery materials technology, and in particular relates to a single-atom doped lithium manganese iron phosphate composite material, its preparation method and application. Background Technology

[0002] Lithium-ion batteries are widely used in many areas of daily life due to their advantages such as being pollution-free, having high specific energy, and long cycle life, including electric vehicles, digital devices, and energy storage power stations. The cathode material is a crucial component of lithium-ion batteries, providing the lithium ions needed for charging and discharging. Therefore, developing high-performance, low-cost cathode materials is key to the development of lithium-ion batteries.

[0003] Lithium manganese iron phosphate (LFP) cathode materials are considered to combine the advantages of both lithium iron phosphate (LFP) and lithium manganese phosphate (LMP), exhibiting a high discharge platform and energy density. More importantly, LFP demonstrates excellent low-temperature performance. However, LFP materials have very low electronic conductivity, only 10⁻⁶. -10 The low S / cm ratio results in a relatively low discharge specific capacity and poor rate performance. Furthermore, manganese dissolves during cycling, leading to poor capacity retention in the cathode material and the emergence of the Jan-Taylor effect, also known as Jan-Taylor deformation. The asymmetric occupation of electrons in degenerate orbitals distorts the molecular geometry, reducing molecular symmetry and orbital degeneracy, further lowering the system's energy. All of these factors limit the development of lithium manganese iron phosphate cathode materials. Summary of the Invention

[0004] The purpose of this application is to provide a single-atom doped lithium manganese iron phosphate composite material and its preparation method, as well as a positive electrode and a secondary battery, which aims to solve to some extent the problems of low electronic conductivity, poor rate performance and low capacity retention of lithium manganese iron phosphate positive electrode materials.

[0005] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows:

[0006] In a first aspect, this application provides a single-atom-doped lithium manganese iron phosphate composite material, the single-atom-doped lithium manganese iron phosphate composite material comprising a lithium manganese iron phosphate core and an outer shell layer covering the outer surface of the core, the outer shell layer comprising carbon material and metal single atoms doped in the carbon material.

[0007] Secondly, this application provides a method for preparing a single-atom-doped lithium manganese iron phosphate composite material, comprising the following steps:

[0008] According to the stoichiometric ratio of the elements in the lithium manganese iron phosphate to be prepared, a lithium source, an iron source, a manganese source, and a phosphorus source are obtained, and the lithium source, the iron source, the manganese source, the phosphorus source, a carbon source, and an organic solvent are mixed to form a slurry;

[0009] After the mixed slurry is dried and ground, it is subjected to a first calcination treatment in an inert atmosphere to obtain carbon-coated lithium manganese iron phosphate.

[0010] The carbon-coated lithium manganese iron phosphate was dispersed in a metal salt solution and mixed. The dried solid product was then subjected to a second calcination treatment in a reducing atmosphere to obtain a single-atom-doped lithium manganese iron phosphate composite material.

[0011] Thirdly, this application provides a positive electrode sheet, wherein the positive electrode active material in the positive electrode sheet includes the above-mentioned single-atom doped lithium manganese iron phosphate composite material, or the single-atom doped lithium manganese iron phosphate composite material prepared by the above method.

[0012] Fourthly, this application provides a secondary battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises the aforementioned positive electrode sheet.

[0013] The first aspect of this application provides a single-atom-doped lithium manganese iron phosphate composite material, comprising a lithium manganese iron phosphate core and a carbon material shell layer doped with metal single atoms covering the outer surface of the core. On one hand, the carbon material coating in the shell layer effectively improves the electrical conductivity of the lithium manganese iron phosphate. On the other hand, when undoped with metal single atoms, the charge of Mn atoms in the lithium manganese iron phosphate core is uniformly distributed. When metal single atoms are doped into the carbon coating layer, surface electrons tend to accumulate around the doped metal single atoms. These electron-rich metal single atoms can act as electron donors, promoting the diffusion of protons and lithium ions in the electrolyte. Therefore, the doping of metal single atoms can effectively suppress the dissolution of manganese in the lithium manganese iron phosphate core, providing suitable sites for lithium nucleation, and can also regulate the chemical properties of the carbon material surface and reduce the nucleation overpotential, significantly reducing the lithium-ion migration barrier and increasing the lithium-ion migration rate. Through the effect of the carbon material shell layer doped with metal single atoms, the comprehensive performance of the composite material, such as structural stability, thermal stability, and electrical conductivity, is effectively improved. Applying single-atom-doped lithium manganese iron phosphate composite materials to lithium-ion batteries can effectively improve the electrochemical performance of lithium-ion batteries, such as rate performance, cycle performance, discharge specific capacity, and low-temperature performance.

[0014] The second aspect of this application provides a method for preparing a single-atom-doped lithium manganese iron phosphate composite material. According to the stoichiometric ratio of the elements in the lithium manganese iron phosphate to be prepared, a lithium source, iron source, manganese source, phosphorus source, carbon source, and organic solvent are mixed into a slurry. The slurry is then dried and ground to refine and homogenize the particle size. A first calcination treatment is then performed in an inert atmosphere, causing the lithium, iron, manganese, and phosphorus sources in the material to be initially converted into lithium manganese iron phosphate material, and the carbon source is initially carbonized to form a carbon material coating layer, resulting in carbon-coated lithium manganese iron phosphate. The carbon-coated lithium manganese iron phosphate is dispersed in a metal salt solution for mixing, allowing the metal salt in the solution to be uniformly and fully adsorbed into the carbon coating layer. In the dried solid product, the metal salt is stably and uniformly adsorbed in the carbon coating layer. The solid product undergoes a second calcination treatment in a reducing atmosphere. This process reduces the metal salts adsorbed in the carbon coating layer into in-situ metal single atoms, which are then loaded into the carbon coating layer, forming metal single-atom doping. Furthermore, the secondary high-temperature calcination further enhances the graphitization degree of the carbon material and improves the structural stability of the lithium manganese iron phosphate core. The method for preparing single-atom-doped lithium manganese iron phosphate composite materials provided in this application is simple, mild, and suitable for large-scale industrial production and application. The carbon coating layer significantly improves the electronic conductivity of the lithium manganese iron phosphate material, while the doping of metal single atoms effectively suppresses manganese dissolution, provides suitable sites for lithium nucleation, lowers the energy barrier for lithium-ion migration, and increases the lithium-ion migration rate. Ultimately, this improves the discharge specific capacity, low-temperature performance, cycle stability, and rate performance of the lithium-ion battery.

[0015] The positive electrode sheet provided in the third aspect of this application includes the aforementioned single-atom-doped lithium manganese iron phosphate composite material as the positive electrode active material. This single-atom-doped lithium manganese iron phosphate composite material, through its carbon material outer shell layer doped with metal single atoms, effectively improves the comprehensive performance of the composite material, including structural stability, thermal stability, and electrical conductivity. This, in turn, enhances the electrochemical performance of the positive electrode sheet in lithium-ion batteries, such as rate performance, cycle performance, discharge specific capacity, and low-temperature performance.

[0016] The secondary battery provided in the fourth aspect of this application includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes the aforementioned positive electrode sheet, which has good rate performance, cycle performance, discharge specific capacity, and low-temperature performance, thereby improving the electrochemical performance of the secondary battery, such as rate performance, cycle performance, and energy density. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic flowchart of the preparation method of single-atom doped lithium manganese iron phosphate composite material provided in the embodiments of this application;

[0019] Figure 2 This is a scanning electron microscope (SEM) image of the lithium manganese iron phosphate composite cathode material with a Co single-atom doped carbon coating provided in Example 1 of this application;

[0020] Figure 3 This is a high-resolution transmission electron microscope (STEM-HRTEM) image of the Co single-atom doped carbon-coated lithium manganese iron phosphate composite cathode material provided in Example 1 of this application under dark field.

[0021] Figure 4 This is the X-ray diffraction (XRD) pattern of the Co single-atom doped carbon-coated lithium manganese iron phosphate composite cathode material provided in Example 1 of this application;

[0022] Figure 5 These are the charge-discharge curves of coin cells prepared with the cathode materials provided in Example 1 and Comparative Example 1 of this application at a rate of 0.1C. Detailed Implementation

[0023] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0024] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0025] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b or c", or "at least one of a, b and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0026] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0027] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms "a" and "the" as used in the embodiments of this application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0028] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass in the embodiments of this application can be a well-known unit of mass in the chemical industry, such as μg, mg, g, or kg.

[0029] The terms "first" and "second" are used for descriptive purposes only, to distinguish objects, such as substances, from one another, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. For example, without departing from the scope of the embodiments of this application, "first XX" may also be referred to as "second XX," and similarly, "second XX" may also be referred to as "first XX." Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of that feature.

[0030] The first aspect of this application provides a single-atom-doped lithium manganese iron phosphate composite material, which includes a lithium manganese iron phosphate core and an outer shell layer covering the outer surface of the core. The outer shell layer includes carbon material and metal single atoms doped in the carbon material.

[0031] The first aspect of this application provides a single-atom-doped lithium manganese iron phosphate composite material, comprising a lithium manganese iron phosphate core and a carbon material shell layer doped with metal single atoms covering the outer surface of the core. On one hand, the carbon material coating in the shell layer effectively improves the electrical conductivity of lithium manganese iron phosphate. On the other hand, when undoped with metal single atoms, the charge of Mn atoms in the lithium manganese iron phosphate core is uniformly distributed. When metal single atoms are doped into the carbon coating layer, surface electrons tend to accumulate around the doped metal single atoms. These electron-rich metal single atoms can act as electron donors, promoting the diffusion of protons and lithium ions in the electrolyte. Therefore, the doping of metal single atoms can effectively suppress the dissolution of manganese in the lithium manganese iron phosphate core, providing suitable sites for lithium nucleation, and can also regulate the chemical properties of the carbon material surface and reduce the nucleation overpotential, significantly reducing the lithium-ion migration barrier and increasing the lithium-ion migration rate. Therefore, the single-atom-doped lithium manganese iron phosphate composite material provided in this application, through the effect of the carbon material shell layer doped with metal single atoms, effectively improves the comprehensive performance of the composite material, including structural stability, thermal stability, and electrical conductivity. Applying single-atom-doped lithium manganese iron phosphate composite materials to lithium-ion batteries can effectively improve the electrochemical performance of lithium-ion batteries, such as rate performance, cycle performance, discharge specific capacity, and low-temperature performance.

[0032] In some possible implementations, the mass percentage of metal single atoms in the outer shell layer is 0.01%–3%. This mass percentage of metal single atoms in the outer shell layer can better suppress manganese dissolution in the lithium manganese iron phosphate core, provide suitable sites for lithium nucleation, reduce the lithium-ion migration barrier, and improve the lithium-ion migration rate. In some specific embodiments, the mass percentage of metal single atoms in the outer shell layer can be 0.01–0.1%, 0.1–0.5%, 0.5–1%, 1–2%, 2–3%, etc.

[0033] In some possible implementations, the metal single atom includes at least one of iron, cobalt, nickel, copper, silver, chromium, manganese, iridium, ruthenium, magnesium, palladium, platinum, bismuth, and tin. Doping these metals into the carbon outer shell of the composite material in single-atom form can suppress the dissolution of core manganese, lower the lithium-ion migration barrier, and increase the lithium-ion migration rate.

[0034] In some possible implementations, the outer shell layer in the single-atom doped lithium manganese iron phosphate composite material has a mass percentage of 0.1% to 3.3%. This mass percentage of the outer shell layer can effectively improve the overall performance of the single-atom doped lithium manganese iron phosphate composite material, such as structural stability, thermal stability, and electrical conductivity, while ensuring the capacity of the composite material and avoiding a reduction in the overall capacity of the composite material due to an excessively high proportion of the outer shell layer. In some specific embodiments, the mass percentage of the outer shell layer in the single-atom doped lithium manganese iron phosphate composite material can be 0.1% to 0.5%, 0.5% to 1%, 1% to 2%, 2% to 3%, 3% to 3.3%, etc.

[0035] In some possible implementations, the average particle size of the single-atom-doped lithium manganese iron phosphate composite material is 0.1–10 μm. The single-atom-doped lithium manganese iron phosphate composite material of this application has a wide particle size range, allowing for flexible adjustment of the particle size according to actual application requirements, thus better meeting different application needs. In some specific embodiments, the average particle size of the single-atom-doped lithium manganese iron phosphate composite material can be 0.1–0.5 μm, 0.5–1 μm, 1–2 μm, 2–3 μm, 3–4 μm, 4–5 μm, 5–6 μm, 6–7 μm, 7–8 μm, 8–9 μm, 9–10 μm, etc.

[0036] As attached Figure 1 As shown, the single-atom doped lithium manganese iron phosphate composite material provided in the above embodiments of this application can be prepared by the methods in the following embodiments.

[0037] The second aspect of this application provides a method for preparing a single-atom-doped lithium manganese iron phosphate composite material, comprising the following steps:

[0038] S10. Obtain lithium source, iron source, manganese source and phosphorus source according to the stoichiometric ratio of the elements in the lithium manganese iron phosphate to be prepared, and prepare a mixed slurry by combining lithium source, iron source, manganese source and phosphorus source with carbon source and organic solvent;

[0039] S20. After drying and grinding the mixed slurry, a first calcination treatment is carried out in an inert atmosphere to obtain carbon-coated lithium manganese iron phosphate;

[0040] S30. Carbon-coated lithium manganese iron phosphate is dispersed in a metal salt solution and mixed. The dried solid product is then subjected to a second calcination treatment in a reducing atmosphere to obtain a single-atom-doped lithium manganese iron phosphate composite material.

[0041] The second aspect of this application provides a method for preparing a single-atom-doped lithium manganese iron phosphate composite material. According to the stoichiometric ratio of the elements in the lithium manganese iron phosphate to be prepared, a lithium source, iron source, manganese source, phosphorus source, carbon source, and organic solvent are mixed into a slurry. This slurry is then dried and ground to refine and homogenize the particle size. A first calcination treatment is then performed in an inert atmosphere, causing the lithium, iron, manganese, and phosphorus sources in the material to be initially converted into lithium manganese iron phosphate material, and the carbon source to be initially carbonized to form a carbon material coating layer, resulting in carbon-coated lithium manganese iron phosphate. The carbon-coated lithium manganese iron phosphate is dispersed in a metal salt solution for mixing, allowing the metal salt in the solution to be uniformly and fully adsorbed into the carbon coating layer. In the dried solid product, the metal salt is stably and uniformly adsorbed in the carbon coating layer. The solid product undergoes a second calcination treatment in a reducing atmosphere. This process reduces the metal salts adsorbed in the carbon coating layer into metal single atoms, which are then situ-loaded into the carbon coating layer, forming metal single-atom doping. Furthermore, the secondary high-temperature calcination further enhances the graphitization degree of the carbon material and improves the structural stability of the lithium manganese iron phosphate core. The method for preparing single-atom-doped lithium manganese iron phosphate composite materials provided in this application is simple, operates under mild conditions, and is suitable for large-scale industrial production and application. The carbon coating layer significantly improves the electronic conductivity of the lithium manganese iron phosphate material, while the doping of metal single atoms effectively suppresses manganese dissolution, provides suitable sites for lithium nucleation, lowers the energy barrier for lithium-ion migration, and increases the lithium-ion migration rate. Ultimately, this improves the discharge specific capacity, low-temperature performance, cycle stability, and rate performance of the lithium-ion battery.

[0042] In step S10 above, lithium, iron, manganese, and phosphorus sources are obtained according to the stoichiometric ratio of the elements in the lithium manganese iron phosphate to be prepared, and the lithium, iron, manganese, and phosphorus sources are mixed with a carbon source and an organic solvent to form a slurry. In some embodiments, the general chemical formula of lithium manganese iron phosphate can be represented as LiMn 1-x Fe x PO4, where 0 < x < 1.

[0043] In some specific embodiments, according to LiMn 1-x Fe x After obtaining lithium, iron, manganese and iron sources in PO4 according to the elemental stoichiometry, they are mixed with carbon source and organic solvent, and then transferred to ball mill jar for ball milling. The ball milling speed is 300 to 500 r / min and the ball milling time is 2 to 12 h to ensure that the raw material components are fully mixed and uniform, so as to prepare a mixed slurry.

[0044] In some possible implementations, the molar ratio of lithium source, iron source, manganese source, phosphorus source and carbon source is 1:(0.1~0.9):(0.1~0.9):(0.5~3.1):(0.01~0.9). Under this ratio, the prepared lithium manganese iron phosphate has better electrochemical properties such as specific capacity, electronic conductivity and rate performance. At the same time, the carbon coating layer formed by the carbon source can better improve the electronic conductivity of the composite material.

[0045] In some possible implementations, the organic solvent includes at least one of methanol, ethanol, glycerol, isopropanol, oleylamine, oleic acid, toluene, aniline, octadecylamine, chloroform, carbon tetrachloride, dimethyl sulfoxide, dimethylformamide, acetone, ethylenediamine, acetaldehyde, formic acid, and acetic acid. These organic solvents have good dissolution / dispersion effects on lithium, iron, manganese, phosphorus, and carbon sources, which is conducive to the full contact of each raw material component to form a uniformly mixed material, and facilitates the full and stable reaction of each material in the subsequent calcination process.

[0046] In some possible implementations, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, lithium formate, lithium silicate, lithium sulfate, lithium phosphate, lithium oxalate, lithium octanoate, lithium citrate, lithium salicylate, lithium orthosilicate, lithium permanganate, lithium trifluoroacetate, lithium acetoacetate, lithium difluorophosphate, lithium hexafluorophosphate, lithium benzoate, lithium metaphosphate, lithium pyruvate, lithium acetate, lithium fluoride, lithium bromide, lithium methoxide, lithium ethanol, lithium oxide, lithium nitride, and lithium sulfide.

[0047] In some possible implementations, the iron source includes at least one of ferrous oxalate, ferrous nitrate, ferrous sulfide, ferrous sulfate, ferrous phosphate, ferrous iodide, ferrous fluoride, ferrous bromide, ferrous acetylacetone, ferrous gluconate, ferrous chloride, and ferrous acetate.

[0048] In some possible implementations, the manganese source includes at least one of manganese carbonate, manganese fluoride, manganese nitride, manganese fluoride, manganese bromide, manganese chloride, manganese carbide, manganese phosphide, potassium permanganate, potassium manganate, manganese acetate, manganese nitrate, manganese phosphate, manganese dihydrogen phosphate, manganese oxalate, manganese pentacarbonyl, manganese decacarbonyl, manganese sulfate, manganese acetate, manganese acetylacetone, and manganese pyrophosphate.

[0049] In some possible implementations, the phosphorus source includes at least one of sodium pyrophosphate, sodium phosphite, sodium metaphosphate, sodium tripolyphosphate, sodium hexafluorophosphate, ammonium hypophosphite, ammonium polyphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium hexafluorophosphate, ammonium phosphate, potassium hypophosphite, potassium pyrophosphate, tripotassium phosphate, potassium phosphite, potassium metaphosphate, and potassium hexafluorophosphate.

[0050] The lithium, iron, manganese, and phosphorus sources used in the above embodiments of this application all have good solubility and can react quickly and efficiently to generate lithium manganese iron phosphate material during the subsequent calcination process.

[0051] In some possible implementations, the carbon source includes at least one of glucose, sucrose, citric acid, ascorbic acid, starch, phenolic resin, commercial toner, carbon nanotubes, graphene, acetylene black, and carbon aerogel. These carbon sources can all be carbonized into carbon materials in a high-temperature inert atmosphere, forming a carbon material shell layer on the outer surface of lithium manganese iron phosphate particles, thereby improving the electronic conductivity of the composite material.

[0052] In step S20 above, the drying conditions for the mixed slurry include: drying under a vacuum atmosphere at a temperature of 60–90°C for 4–24 hours; or, freeze-drying. The solvent in the mixed slurry is thoroughly removed through vacuum drying or freeze-drying. In some specific embodiments, the drying temperature can be 60–70°C, 70–80°C, 80–90°C, etc., and the drying time can be 4–10 hours, 10–15 hours, 15–20 hours, 20–24 hours, etc.

[0053] In some possible implementations, the dried mixed slurry is ground and pulverized to refine the particle size. Specifically, the grinding conditions after drying the mixed slurry include ball milling at a speed of 300–500 r / min for 2–12 hours. In some specific embodiments, the speed can be 300–350 r / min, 350–400 r / min, 400–450 r / min, 450–500 r / min, etc., and the ball milling time can be 2–5 hours, 5–8 hours, 8–10 hours, 10–12 hours, etc.

[0054] In some possible implementations, the conditions for the first calcination treatment include: calcination for 5–18 hours in an inert atmosphere at a temperature of 250–500°C. Under these conditions, the lithium, iron, manganese, and phosphorus sources in the material are initially converted into lithium manganese iron phosphate material, and the carbon source is initially carbonized to form a carbon material coating layer, resulting in carbon-coated lithium manganese iron phosphate. In some specific embodiments, the inert atmosphere includes hydrogen, argon, helium, etc.

[0055] In step S30 above, the carbon-coated lithium manganese iron phosphate is dispersed in a metal salt solution for mixing. In some possible implementations, the mixing step includes: after dispersing the carbon-coated lithium manganese iron phosphate in the metal salt solution, ultrasonically treating it at a power of 300-1000W for 0.5-5 hours, and stirring it at a speed of 200-600 r / min for 3-12 hours; so that the metal salt in the solution is fully and uniformly adsorbed into the carbon coating layer. In some specific embodiments, the ultrasonic power can be 300-500W, 500-800W, 800-1000W, etc., the ultrasonic duration can be 0.5-1h, 1-2h, 2-3h, 3-4h, 4-5h, etc., the stirring speed can be 200-300r / min, 300-400r / min, 400-500r / min, 500-600r / min, etc., and the stirring duration can be 3-5h, 5-8h, 8-10h, 10-12h, etc.

[0056] In some possible implementations, the metal salt in the metal salt solution includes at least one soluble metal salt selected from iron, cobalt, nickel, copper, silver, chromium, manganese, iridium, ruthenium, magnesium, palladium, platinum, bismuth, and tin. These soluble metal salts all exhibit good solubility and can dissolve uniformly and stably in organic solvents. Furthermore, during subsequent reduction and calcination, they can be reduced to single metal atoms. In the doped carbon material coating layer, the dissolution of manganese from the lithium iron phosphate core can be effectively suppressed, providing suitable sites for lithium nucleation. Additionally, the chemical properties of the carbon material surface can be adjusted, and the nucleation overpotential reduced, significantly lowering the lithium-ion migration barrier and increasing the lithium-ion migration rate.

[0057] In some possible implementations, the concentration of the metal salt solution is 0.01–1 mol / L. In the embodiments of this application, the concentration of the metal salt solution should not be too high. A lower metal salt concentration ensures that fewer metal ions are loaded on the carbon layer, and the metal exists in the form of single atoms after high-temperature reduction. If the concentration of the metal salt solution is too high, the metal single atoms will exist in the form of clusters or crystal particles after reduction, reducing the improvement effect on the composite material's ability to inhibit manganese dissolution and lithium ion migration rate. In some specific embodiments, the concentration of the metal salt solution can be 0.01–0.05 mol / L, 0.05–0.1 mol / L, 0.1–0.15 mol / L, 0.1–0.2 mol / L, 0.2–0.3 mol / L, 0.3–0.5 mol / L, 0.5–0.8 mol / L, 0.8–1 mol / L, etc. In some preferred embodiments, the concentration of the metal salt solution is 0.05 to 0.15 mol / L, which better ensures that the metal is loaded into the carbon layer in the form of single atoms.

[0058] In some possible implementations, after mixing, the solid and liquid are separated, and the solid product is dried in an inert atmosphere or vacuum atmosphere at a temperature of 60–100°C for 5–24 hours. Low-temperature drying in an inert atmosphere or vacuum atmosphere is performed to avoid oxidation of the composite material during drying. Organic solvents are removed through low-temperature drying to obtain the dried solid product. In some specific embodiments, the inert atmosphere includes hydrogen, argon, helium, etc.

[0059] In some possible implementations, the conditions for the second calcination treatment include: calcining the dried solid product in a reducing atmosphere at a temperature of 500–950°C and a hydrogen volume content of 5–10% for 2–20 hours. Under these conditions, the second calcination treatment not only reduces the metal salts adsorbed in the carbon coating layer to metal single atoms, which are then loaded into the carbon coating layer, forming metal single-atom doping; but also further enhances the graphitization degree of the carbon material and improves the structural stability of the lithium manganese iron phosphate core. In some specific embodiments, the reducing atmosphere includes hydrogen and nitrogen / argon / helium in volume ratios of 5%:95%, 6%:94%, 7%:93%, 8%:92%, 9%:91%, or 10%:90%; the calcination temperature can be 500–600℃, 600–700℃, 700–800℃, 800–900℃, 900–950℃, etc.; the calcination time can be 2–5h, 5–8h, 8–10h, 10–13h, 13–15h, 15–18h, 18–20h, etc.

[0060] A third aspect of this application provides a positive electrode sheet, wherein the positive electrode active material includes the above-mentioned single-atom doped lithium manganese iron phosphate composite material, or the single-atom doped lithium manganese iron phosphate composite material prepared by the above method.

[0061] The positive electrode sheet provided in the third aspect of this application includes a positive electrode active material comprising the aforementioned single-atom-doped lithium manganese iron phosphate composite material. This single-atom-doped lithium manganese iron phosphate composite material, through a carbon material outer shell layer doped with metal single atoms, effectively improves the comprehensive performance of the composite material, including structural stability, thermal stability, and electrical conductivity. This, in turn, enhances the electrochemical performance of the positive electrode sheet in lithium-ion batteries, such as rate performance, cycle performance, discharge specific capacity, and low-temperature performance.

[0062] In some possible implementations, the positive electrode also includes at least one binder such as polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), or polytetrafluoroethylene (PTFE).

[0063] In some possible implementations, the positive electrode also includes at least one conductive agent selected from carbon black, carbon nanotubes, graphene, etc.

[0064] In some possible implementations, the preparation steps of the positive electrode sheet include: dissolving a single-atom-doped lithium manganese iron phosphate composite material, a binder, and a conductive agent in an organic solvent to form a positive electrode slurry, coating it onto a positive electrode current collector, drying, rolling, and stamping to obtain the positive electrode sheet.

[0065] In some possible implementations, the mass ratio of the single-atom doped lithium manganese iron phosphate composite material (i.e., positive electrode active material), binder, conductive agent and organic solvent in the positive electrode slurry is (70-80):(1-2):(1-2):(20-30).

[0066] A fourth aspect of this application provides a secondary battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode includes the aforementioned positive electrode sheet.

[0067] The secondary battery provided in the fourth aspect of this application includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes the aforementioned positive electrode sheet, which has good rate performance, cycle performance, discharge specific capacity, and low-temperature performance, thereby improving the electrochemical performance of the secondary battery, such as rate performance, cycle performance, and energy density.

[0068] In some possible implementations, the negative electrode of the secondary battery includes, but is not limited to, carbon materials such as graphite, soft carbon (such as coke), and hard carbon, or nitrides, tin-based oxides, tin alloys, and nano-anode materials.

[0069] In some possible implementations, the diaphragm includes at least one material selected from polypropylene fiber, polyacrylonitrile fiber, polyvinyl alcohol formal fiber, poly(ethylene glycol terephthalate), polyethylene terephthalate, polyamide fiber, and poly(p-phenylene terephthalamide).

[0070] In some possible implementations, the electrolyte includes Na-containing electrolytes. + K + NH 4+ An aqueous solution of at least one of the soluble salts.

[0071] To enable those skilled in the art to clearly understand the above-described implementation details and operations, and to demonstrate the significant advancements in the performance of the single-atom-doped lithium manganese iron phosphate composite material, its preparation method, and its application, the following examples illustrate the above technical solutions.

[0072] Example 1

[0073] The specific steps for preparing a single-atom-doped carbon-coated lithium manganese iron phosphate composite cathode material are as follows:

[0074] 1. Weigh 3g lithium carbonate (0.0406mol), 2.4g ferrous sulfate (0.0158mol), 2.7g manganese carbonate (0.0229mol), 5.2g ammonium dihydrogen phosphate (0.0455mol), and 0.5g glucose (0.0037mol) using a balance. Weigh 50mL anhydrous ethanol using a graduated cylinder. Add all ingredients to an agate ball mill jar and ball mill at 400r / min for 5 hours to fully mix and obtain a mixed slurry.

[0075] 2. Place the mixed slurry in a vacuum drying oven at 80°C for 12 hours. Grind the dried product and calcine it in a tube furnace under a nitrogen atmosphere at 370°C for 8 hours. The product is carbon-coated lithium manganese iron phosphate.

[0076] 3. Weigh 10g of carbon-coated lithium manganese iron phosphate product and disperse it in a 0.1mol / L cobalt nitrate solution. Sonicate at 600W for 0.5h, stir at 300r / min for 12h, and then centrifuge to complete solid-liquid separation. Dry the separated solid under vacuum at 80℃ for 5h, and then calcine it in a tube furnace. Pass a hydrogen-argon mixture gas with a volume ratio of 5% / 95% through it. The calcination temperature is 700℃ and the calcination time is 6h. The product is the cobalt single-atom doped carbon-coated lithium manganese iron phosphate composite cathode material, labeled as LMFP@C / Co-SAc.

[0077] Example 2

[0078] The specific steps for preparing a single-atom-doped carbon-coated lithium manganese iron phosphate composite cathode material are as follows:

[0079] 1. Weigh 0.98g lithium hydroxide (0.0411mol), 2.9g ferrous oxalate (0.0201mol), 3.6g manganese nitrate (0.0199mol), 6.1g diammonium hydrogen phosphate (0.0461mol), and 5g graphene powder using a balance. Weigh 60mol isopropanol using a graduated cylinder. Add all of these to an agate ball mill jar and ball mill at 350r / min for 6 hours to fully mix and obtain a mixed slurry.

[0080] 2. Place the mixed slurry in a vacuum drying oven at 70°C for 10 hours. Grind the dried product and calcine it in a tube furnace under a nitrogen atmosphere at 350°C for 6 hours. The product is carbon-coated lithium manganese iron phosphate.

[0081] 3. Weigh 10g of carbon-coated lithium manganese iron phosphate product and disperse it in a 0.1mol / L silver nitrate solution. Sonicate at 600W for 1h, stir at 300r / min for 6h, and then centrifuge to complete solid-liquid separation. Dry the separated solid under vacuum at 80℃ for 5h, and then calcine it in a tube furnace. Pass a hydrogen-argon mixture at a volume ratio of 5% / 95% and calcine it at 900℃ for 3h. The product is the silver single-atom doped carbon-coated lithium manganese iron phosphate composite cathode material, labeled as LMFP@C / Ag-SAc.

[0082] Example 3

[0083] The specific steps for preparing a single-atom-doped carbon-coated lithium manganese iron phosphate composite cathode material are as follows:

[0084] 1. Weigh 2.1g lithium chloride (0.0511mol), 2.8g ferrous chloride (0.0219mol), 5.6g manganese sulfate (0.0369mol), 4.1g ammonium hypophosphite (0.0499mol), and 5g commercial toner (XC-72R) using a balance. Weigh 60mol methanol using a graduated cylinder. Add all ingredients to an agate ball mill jar and ball mill at 400r / min for 4 hours to fully mix and obtain a mixed slurry.

[0085] 2. Place the mixed slurry in a vacuum drying oven at 60°C for 12 hours. Grind the dried product and calcine it in a tube furnace under a nitrogen atmosphere at 400°C for 5 hours. The product is carbon-coated lithium manganese iron phosphate.

[0086] 3. Weigh 10g of carbon-coated lithium manganese iron phosphate product and disperse it in a 0.1mol / L copper chloride solution. Sonicate at 600W for 1h, stir at 300r / min for 12h, and then centrifuge to complete solid-liquid separation. Dry the separated solid under vacuum at 80℃ for 5h, and then calcine it in a tube furnace. Pass a hydrogen-argon mixture at a volume ratio of 5% / 95% and calcine it at 800℃ for 5h. The product is the copper single-atom doped carbon-coated lithium manganese iron phosphate composite cathode material, labeled as LMFP@C / Cu-SAc.

[0087] Example 4

[0088] A single-atom carbon-doped lithium manganese iron phosphate composite cathode material is disclosed. The operation steps of this embodiment are basically the same as those of Embodiment 1, except that the concentration of the cobalt nitrate solution in step 3 is changed from "0.1mol / L" to "0.5mol / L". The product is still a cobalt single-atom carbon-doped lithium manganese iron phosphate composite cathode material, labeled as LMFP@C / Co-SAc.

[0089] Example 5

[0090] A single-atom-doped carbon-coated lithium manganese iron phosphate composite cathode material is disclosed. The operation steps of this embodiment are completely consistent with steps 1 and 2 of embodiment 1. The difference is that the specific operation steps of step 3 are as follows: 10g of carbon-coated lithium manganese iron phosphate product and 50mg of cobalt nitrate powder are weighed and added to an agate mortar and manually ground for 0.5h. Then, it is placed in a tube furnace for calcination. A hydrogen-argon mixed gas with a volume ratio of 5% / 95% is introduced. The calcination temperature is 700℃ and the calcination time is 6h. The product is the cobalt single-atom-doped carbon-coated lithium manganese iron phosphate composite cathode material, labeled as LMFP@C / Co-SAc.

[0091] Comparative Example 1

[0092] A carbon-coated lithium manganese iron phosphate composite cathode material is prepared using the same steps as steps 1 and 2 in Example 1. The product is the carbon-coated lithium manganese iron phosphate cathode material, labeled LMFP@C.

[0093] Furthermore, to verify the embodiments and comparative examples of this application, the following performance tests were conducted:

[0094] 1. The morphology of the cobalt single-atom doped carbon-coated lithium manganese iron phosphate composite cathode material prepared in Example 1 was observed using scanning electron microscopy, as shown in the attached figure. Figure 2 As shown in the SEM image, the prepared composite cathode material has small particle size and high uniformity.

[0095] 2. The morphology of the cobalt single-atom doped carbon-coated lithium manganese iron phosphate composite cathode material prepared in Example 1 was observed under dark field using high-resolution transmission electron microscopy, as shown in the attached figure. Figure 3 As shown in the STEM-HRTEM image, cobalt single atoms are uniformly distributed in the composite cathode material.

[0096] 3. X-ray diffraction tests were performed on the cobalt single-atom doped carbon-coated lithium manganese iron phosphate composite cathode material prepared in Example 1, as shown in the attached figure. Figure 4 As shown in the XRD pattern, the characteristic peaks are obvious, indicating that cobalt atoms are doped into the composite cathode material.

[0097] 4. The carbon content, specific surface area, powder compaction density, and metal single-atom content of the cathode materials prepared in each embodiment and comparative example were tested. The test results are shown in Table 1 below:

[0098] Table 1

[0099]

[0100] As can be seen from the test results in Table 1 above, the carbon-coated lithium manganese iron phosphate composite cathode material doped with metal single atoms prepared in this application embodiment exhibits a better specific surface area and powder compaction density compared with the carbon-coated lithium manganese iron phosphate composite cathode material without metal single atoms in Comparative Example 1, which is more conducive to its application in lithium-ion battery cathode materials to improve the electrochemical performance of the battery.

[0101] 5. The cathode materials prepared in each embodiment and comparative example are applied to lithium-ion batteries to prepare coin cells. The specific steps are as follows:

[0102] ① Preparation of slurry: 23.75g of the positive electrode material prepared in the example or comparative example was added to a 500mL agate ball mill jar along with 0.5g of superconducting carbon black (SP) and 0.75g of binder polyvinylidene fluoride (PVDF). Then, 16g of solvent N-methylpyrrolidone (NMP) was added, and the mixture was ball milled at 360r / min for 4h to obtain the slurry.

[0103] ② Coating of slurry: Adjust the scale of the scraper of the coating machine, and evenly coat the slurry after ball milling onto the aluminum foil. Place the coated electrode in a vacuum drying oven at 130℃ and bake for 3 hours.

[0104] ③ Rolling and stamping: Place the aluminum foil coated with slurry flat in the center of the roller and roll it to press the electrode sheet; then press the rolled electrode sheet with the front side tightly against the perforated area and stamp it sequentially; the compaction density of the electrode sheet is controlled at 2.0~2.4g / cm³. 3 The diameter is 14mm and the thickness is 0.05~0.10mm; the punched electrode is placed in a vacuum drying oven at 130℃ and baked for 3 hours;

[0105] ④ Assemble the button cell battery. In the glove box, assemble the negative electrode shell, spring, steel sheet, lithium sheet, separator, positive electrode sheet and positive electrode shell in sequence. During the process, inject 10μL of electrolyte. Then use a sealing machine to seal the button cell battery to obtain the button cell batteries corresponding to the positive electrode materials of Examples 1 to 3 and Comparative Example 1, respectively.

[0106] Electrochemical performance tests were conducted on the four sets of coin cells: 0.1C discharge capacity, 1C discharge capacity, capacity retention after 200 charge-discharge cycles at 1C, Mn dissolution after 200 cycles, resistivity, and D were measured. Li The test results are shown in Table 2 below:

[0107] Table 2

[0108]

[0109]

[0110] As can be seen from the test results in Table 2 above, compared with the coin cell prepared by the carbon-coated lithium manganese iron phosphate composite cathode material without metal single atoms in Comparative Example 1, the coin cell prepared by the single-atom doped carbon-coated lithium manganese iron phosphate composite cathode material prepared in this application embodiment can effectively suppress manganese dissolution during charging and discharging, improve lithium-ion migration efficiency, greatly enhance cycle stability, and has low resistivity, high 0.1C and 1C discharge capacity, and good rate performance. In particular, the coin cell prepared by the single-atom doped carbon-coated lithium manganese iron phosphate composite cathode material provided in Example 1 has the lowest Mn content detected in the electrolyte after 200 charge-discharge cycles, which is only 346 ppm, and the resistivity is greatly reduced, with a minimum of only 5.6 Ω·cm. The lithium-ion migration rate DLi is 9.6 × 10⁻⁶. -13 Lithium-ion migration is easy, and insertion / extraction is smooth. The highest discharge specific capacity at 0.1C can reach 151.1 mAh / g, and the highest discharge specific capacity at 1C can reach 141.9 mAh / g, which is much higher than that of Comparative Example 1 (122.3 mAh / g). Furthermore, the capacity retention rate after 200 cycles reaches 99.1%, significantly better than the capacity retention rate of Comparative Example 1 (70.3%). Comparison of Examples 1 and 4 shows that when the metal single-atom content is 0.15%, the coin cell prepared with the composite cathode material exhibits lower resistivity, higher lithium-ion migration rate, better rate performance, and lower manganese dissolution rate. Comparison of Examples 1 and 5 shows that the composite material prepared in Example 5 through solid-phase doping leads to uneven distribution of metal single atoms, resulting in increased resistivity, decreased lithium-ion migration rate, decreased rate performance, and a significantly increased manganese dissolution rate in the coin cell, thus reducing the electrochemical performance of the battery. In addition, the charge-discharge curves of the coin cells prepared in Example 1 (labeled as Example B1) and Comparative Example 1 (labeled as Comparative Example B1) at a 0.1C rate are attached. Figure 5 As shown, the coin cell prepared in Example 1 of this application exhibits better rate performance.

[0111] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A single-atom-doped lithium manganese iron phosphate composite material, characterized in that, The single-atom-doped lithium manganese iron phosphate composite material comprises a lithium manganese iron phosphate core and an outer shell layer covering the outer surface of the core. The outer shell layer comprises carbon material and metal single atoms doped in the carbon material. In the single-atom-doped lithium manganese iron phosphate composite material, the mass percentage of the outer shell layer is 0.1% to 3.3%; the mass percentage of the metal single atoms in the outer shell layer is 0.01% to 3%; the metal single atoms include at least one selected from iron, cobalt, nickel, copper, silver, chromium, manganese, iridium, ruthenium, magnesium, palladium, platinum, bismuth, and tin. The preparation of the single-atom-doped lithium manganese iron phosphate composite material includes the following steps: after preparing carbon-coated lithium manganese iron phosphate, dispersing the carbon-coated lithium manganese iron phosphate in a metal salt solution and ultrasonically treating it with a power of 300-1000 W for 0.5-5 h, mixing it at a speed of 200-600 r / min for 3-12 h, and calcining the dried solid product in a reducing atmosphere at a temperature of 500-950℃ and a hydrogen volume content of 5-10% for 2-20 h to obtain the single-atom-doped lithium manganese iron phosphate composite material; the concentration of the metal salt solution is 0.05-0.15 mol / L.

2. The single-atom-doped lithium manganese iron phosphate composite material as described in claim 1, characterized in that, The average particle size of the single-atom doped lithium manganese iron phosphate composite material is 0.1 ~ 10 μm.

3. A method for preparing a single-atom-doped lithium manganese iron phosphate composite material as described in any one of claims 1 to 2, characterized in that, Includes the following steps: According to the stoichiometric ratio of the elements in the lithium manganese iron phosphate to be prepared, a lithium source, an iron source, a manganese source, and a phosphorus source are obtained, and the lithium source, the iron source, the manganese source, the phosphorus source, a carbon source, and an organic solvent are mixed to form a slurry; After the mixed slurry is dried and ground, it is subjected to a first calcination treatment in an inert atmosphere to obtain carbon-coated lithium manganese iron phosphate. The carbon-coated lithium manganese iron phosphate was dispersed in a metal salt solution and ultrasonically treated with a power of 300-1000 W for 0.5-5 h, then mixed at a speed of 200-600 r / min for 3-12 h. The dried solid product was then subjected to a second calcination treatment at a temperature of 500-950℃ and a hydrogen volume content of 5-10% for 2-20 h to obtain a single-atom doped lithium manganese iron phosphate composite material.

4. The method for preparing the single-atom doped lithium manganese iron phosphate composite material as described in claim 3, characterized in that, The molar ratio of the lithium source, the iron source, the manganese source, the phosphorus source and the carbon source is 1 : (0.1 ~ 0.9): (0.1 ~ 0.9): (0.5 ~ 3.1): (0.01 ~ 0.9).

5. The method for preparing the single-atom doped lithium manganese iron phosphate composite material as described in claim 4, characterized in that, The drying conditions for the mixed slurry include: drying under vacuum at a temperature of 60-90°C for 4-24 hours; or, freeze drying. And / or, the grinding conditions after the mixed slurry is dried include: ball milling for 2 to 12 hours at a rotation speed of 300 to 500 r / min; And / or, the conditions for the first calcination treatment include: calcination for 5 to 18 hours in an inert atmosphere at a temperature of 250 to 500 °C.

6. The method for preparing the single-atom doped lithium manganese iron phosphate composite material according to any one of claims 3 to 5, characterized in that, The metal salt in the metal salt solution includes at least one soluble metal salt selected from iron salt, cobalt salt, nickel salt, copper salt, silver salt, chromium salt, manganese salt, iridium salt, ruthenium salt, magnesium salt, palladium salt, platinum salt, bismuth salt, and tin salt.

7. The method for preparing the single-atom doped lithium manganese iron phosphate composite material as described in claim 6, characterized in that, After the mixing process, the solid and liquid are separated, and the solid product is dried in an inert atmosphere or vacuum atmosphere at a temperature of 60-100 °C for 5-24 h.

8. A positive electrode plate, characterized in that, The positive electrode active material in the positive electrode sheet includes the single-atom doped lithium manganese iron phosphate composite material as described in any one of claims 1 to 2, or the single-atom doped lithium manganese iron phosphate composite material prepared by the method described in any one of claims 3 to 7.

9. A secondary battery, characterized in that, The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises a positive electrode sheet as described in claim 8.

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