Lithium intercalation phosphate-based composite material and method for producing the same, secondary battery

Abstract

The application belongs to the technical field of battery materials, and particularly relates to a lithium intercalation phosphate composite material and a preparation method thereof, and a secondary battery. The lithium intercalation phosphate composite material comprises a lithium intercalation phosphate core and a coating layer coated on the outer surface of the core; the coating layer comprises carbon material and black phosphorus, and the black phosphorus and the carbon material form a mosaic and overlapping composite structure. The coating layer of the lithium intercalation phosphate composite material provided by the application can improve the stability of the core phosphate, effectively improve the electrical conductivity and the carrier transport rate, and through the synergistic effect of the core and the coating layer, the lithium intercalation phosphate composite material has the characteristics of high capacity, high rate performance, high cycle stability and the like.

Description

Technical Field

[0001] This application belongs to the field of battery materials technology, and particularly relates to a lithium-intercalated phosphate composite material and its preparation method, as well as a secondary battery. Background Technology

[0002] Secondary batteries can reversibly convert chemical energy into electrical energy, making them an ideal carrier for human energy utilization and storage. Since the advent of lithium-ion batteries, they have been widely used due to their advantages such as high energy density, good cycle performance, and environmental friendliness. Currently, lithium-ion batteries are used in various fields such as 3C electronics, new energy vehicles, and smart grids. Among them, the cathode material is the key to affecting battery performance, often determining the battery's cycle life, energy density, and power density.

[0003] Currently, the main commercially available cathode materials include layered LiCoO2, layered ternary materials, spinel-structured LiMn2O4, and olivine-structured LiFePO4. The cathode materials for lithium-ion batteries used in new energy vehicles are primarily lithium iron phosphate and ternary materials. Among these, LiFePO4 has advantages in safety and cycle performance, but is inferior in energy density and power density, while ternary materials are the opposite. With the continuous development of the new energy vehicle sector, increasingly higher requirements are being placed on the energy density, power density, cycle performance, and safety of lithium-ion batteries.

[0004] LiMnPO4, with a structure similar to LiFePO4, has a redox potential of approximately 4.1 V and exhibits a higher energy density than LiFePO4 at the same specific capacity. However, the poor conductivity of LiMnPO4 hinders electron and ion migration, making it difficult to replace LiFePO4. Lithium manganese iron phosphate (LFP), a solid solution formed by replacing some Mn with Fe, combines the advantages of both LiFePO4 and LiMnPO4, making it a promising cathode material for commercialization. However, the cycle stability and rate performance of LFP still require further improvement. Surface coating is a commonly used method for modifying electrode materials. The chemical composition, component distribution, thickness, and coverage of the surface coating layer all affect the performance of the electrode material. Therefore, current modification methods have limited effect on improving the electrochemical performance of phosphate-based materials, such as cycle stability and rate performance, and the performance of phosphate-based materials still needs further improvement. Summary of the Invention

[0005] The purpose of this application is to provide a lithium-intercalated phosphate composite material and its preparation method, as well as a secondary battery, which aims to address to some extent the problem that the cycle stability and rate performance of existing lithium-intercalated phosphate systems need to be further improved.

[0006] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows:

[0007] In a first aspect, this application provides a lithium-intercalated phosphate composite material, comprising a lithium-intercalated phosphate core and a coating layer covering the outer surface of the core; the coating layer comprises carbon material and black phosphorus, wherein the black phosphorus and the carbon material form an intercalated and overlapping composite structure.

[0008] In some possible implementations, the embedded and overlapping composite structure includes: a portion of the black phosphorus being embedded in the carbon material in the form of nano-black phosphorus sheets to form a composite coating layer; and a portion of the black phosphorus forming a multi-layered overlapping composite coating layer with the carbon material.

[0009] In some possible implementations, in the coating layer, a portion of the black phosphorus replaces carbon atoms in the carbon material in the form of atomic clusters or single atoms to form phosphorus-doped carbon material.

[0010] In some possible implementations, the lithium-intercalated phosphate includes at least one of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, and lithium vanadium phosphate.

[0011] In some possible implementations, the nano-black phosphorus sheet includes: a small-sized sheet with an average thickness of 1-5 nm and an average length / width of 5-15 nm, and a large-sized sheet with an average thickness of 5-15 nm and an average length / width of 15-1000 nm.

[0012] In some possible implementations, the black phosphorus content in the lithium-intercalated phosphate composite material is 0.1 to 3 wt%.

[0013] In some possible implementations, the coating layer in the lithium-intercalated phosphate composite material has a mass percentage content of 1 to 5 wt%.

[0014] In some possible implementations, the black phosphorus content in the coating layer is 10–60 wt%.

[0015] In some possible implementations, the primary particle size of the lithium-intercalated phosphate composite material is 30–500 nm, and the secondary particle size is 0.3–10 μm.

[0016] In some possible implementations, the coating thickness in the lithium-intercalated phosphate composite material is 0.5–8 nm.

[0017] Secondly, this application provides a method for preparing a phosphate-based composite material, comprising the following steps:

[0018] According to the stoichiometric ratio of the elements in the lithium intercalated phosphate, the raw material components are obtained and then mixed and ground with red phosphorus and organic grinding aid in an inert atmosphere to obtain a mixed precursor.

[0019] The mixed precursor is sintered in an inert atmosphere to obtain a core-shell structured lithium-intercalated phosphate composite material, wherein the core is a lithium-intercalated phosphate and the coating layer includes carbon material and black phosphorus forming an intercalated and overlapping composite structure.

[0020] In some possible implementations, the sintering process includes: performing a first sintering process on the mixed precursor, followed by cooling and then performing a second sintering process; wherein the temperature of the first sintering process is lower than the temperature of the second sintering process.

[0021] In some possible implementations, the conditions for the mixed grinding process include: ball milling for 6 to 24 hours in an inert atmosphere with a ball-to-material ratio of (15 to 50):1 and a rotation speed of 500 to 1200 rpm.

[0022] In some possible implementations, the conditions for the first sintering treatment include: heating to 200-500°C at a rate of 1-5°C / min under an inert atmosphere and holding at that temperature for 3-15 hours.

[0023] In some possible implementations, the conditions for the second sintering treatment include: heating to 500–800°C at a rate of 1–5°C / min under an inert atmosphere and holding at that temperature for 3–15 hours.

[0024] In some possible implementations, the hybrid grinding process includes planetary ball milling or oscillating ball milling.

[0025] In some possible implementations, the red phosphorus content in the mixed precursor is 0.1 to 3 wt%.

[0026] In some possible implementations, the organic grinding aid in the mixed precursor is 0.5 to 6 wt% by mass.

[0027] In some possible implementations, the organic grinding aid includes at least one of urea, starch, glucose, sucrose, fructose, and citric acid.

[0028] Thirdly, this application provides a secondary battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte; wherein the positive electrode contains the above-mentioned lithium-intercalated phosphate composite material or the lithium-intercalated phosphate composite material prepared by the above method.

[0029] The first aspect of this application provides a lithium-intercalated phosphate composite material, comprising a lithium-intercalated phosphate core and a coating layer covering the outer surface of the core. The lithium-intercalated phosphate in the core integrates the stability of the olivine crystal structure. The coating layer includes carbon material and black phosphorus forming an interlocking and overlapping composite structure. Black phosphorus is the least reactive of the phosphorus allotropes, its lattice consists of linked six-membered rings, and it has trigonal, cubic, and orthorhombic crystal structures. Black phosphorus flakes have a layered structure similar to graphite, exhibiting excellent conductivity and high ion / electron transport rates. The physical doping of the coating layer with the carbon material to form an interlocking and overlapping composite structure, and the chemical doping of black phosphorus existing as atomic clusters or individual atoms with the carbon material, can both improve the stability of the phosphate core and effectively improve the conductivity and carrier transport rate of the coating layer, thereby improving the rate performance and cycle stability of the phosphate composite material.

[0030] The second aspect of this application provides a method for preparing a lithium-intercalated phosphate composite material. After obtaining the lithium-intercalated phosphate raw material components, it is mixed and ground with red phosphorus and an organic grinding aid in an inert atmosphere. This mixing and grinding process ensures thorough and uniform mixing of the raw material components. Furthermore, the localized high temperature and pressure during the mixing and grinding process initially converts the red phosphorus into black phosphorus. With the assistance of the organic grinding aid, the black phosphorus is exfoliated to produce two-dimensional black phosphorus nanosheets or atomic clusters. The inert atmosphere is used to prevent the unstable red phosphorus from being oxidized during the mixing and grinding process. The mixed precursor is then sintered in an inert atmosphere. Some of the red phosphorus remaining from the ball milling process is completely converted into black phosphorus during sintering, while the organic grinding aid is converted into carbon material during sintering. This carbon material forms an embedded and overlapping composite structure with the black phosphorus, jointly coating the surface of the phosphate particles to obtain a core-shell structured lithium-intercalated phosphate composite material. The prepared composite material exhibits higher capacity, conductivity, and rate performance, among other electrochemical properties. Moreover, this preparation process is simple, low-cost, and has minimal environmental impact, providing an optional method for the industrial preparation of high-rate-performance phosphate electrode materials.

[0031] The secondary battery provided in the third aspect of this application includes a positive electrode comprising the aforementioned lithium-intercalated phosphate composite material. This lithium-intercalated phosphate composite material has a core-shell structure, with the core being a lithium-intercalated phosphate compound. The coating layer comprises carbon material and black phosphorus forming an intercalated and overlapping composite structure, exhibiting characteristics such as high capacity, high rate performance, and high cycle stability. Therefore, it can improve the energy density, cycle stability, and rate performance of the secondary battery. Attached Figure Description

[0032] 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.

[0033] Figure 1 This is a schematic flowchart of the preparation method of the lithium-intercalated phosphate composite material provided in the embodiments of this application;

[0034] Figure 2 This is the X-ray diffraction pattern of the lithium manganese iron phosphate composite material co-coated with black phosphorus and carbon provided in Example 1 of this application;

[0035] Figure 3 This is a scanning electron microscope (SEM) image of the lithium manganese iron phosphate composite material co-coated with black phosphorus and carbon provided in Example 1 of this application.

[0036] Figure 4 The capacity-voltage curves of coin cells prepared from the black phosphorus and carbon co-coated lithium manganese iron phosphate composite material provided in Example 1 of this application and the carbon-coated lithium manganese iron phosphate composite material prepared in Comparative Example 1 are shown at a rate of 0.1C.

[0037] Figure 5 These are specific capacity test graphs of coin cells made from lithium manganese iron phosphate composite materials prepared in Examples 1 and 2 of this application at different rates. Detailed Implementation

[0038] 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. Lithium manganese iron phosphate is used as an example in these specific embodiments.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] The first aspect of this application provides a lithium-intercalated phosphate-based composite material, characterized in that it includes a lithium-intercalated phosphate core and a coating layer covering the outer surface of the core; the coating layer includes carbon material and black phosphorus, and the black phosphorus and carbon material form an intercalated and overlapping composite structure.

[0046] The first aspect of this application provides a lithium-intercalated phosphate composite material comprising a lithium manganese iron phosphate core and a coating layer covering the outer surface of the core. The lithium-intercalated phosphate core integrates the stability of the olivine crystal structure. The coating layer comprises carbon material and black phosphorus forming an interlocking and overlapping composite structure. Black phosphorus is the least reactive of the phosphorus allotropes, its lattice composed of linked six-membered rings, exhibiting trigonal, cubic, and orthorhombic crystal structures. Black phosphorus flakes have a layered structure similar to graphite, possessing excellent conductivity and high ion / electron transport rates. The black phosphorus and carbon material in the coating layer form physical doping, such as interpenetrating and embedding to jointly form the composite coating layer; and black phosphorus existing as atomic clusters or individual atoms can form chemical doping with the carbon material. This not only improves the stability of the lithium manganese iron phosphate core but also effectively improves the conductivity and carrier transport rate of the coating layer, thereby enhancing the rate performance and cycle stability of the lithium-intercalated phosphate composite material. Therefore, the lithium-intercalated phosphate composite material provided in this application embodiment has the characteristics of high capacity, high rate performance, and high cycle stability through the synergistic effect of the core and the coating layer.

[0047] In some possible implementations, the embedded and overlapping composite structures include: partially embedded black phosphorus in the form of nano-sized black phosphorus flakes to form a composite coating layer within carbon material. In this case, the black phosphorus flakes have a small diameter, such as less than 10 nm. These small-diameter flakes can be nested in a "mosaic" pattern within the carbon material, forming a composite material coating the surface of the core lithium manganese iron phosphate, improving the composite material's cycle stability, rate performance, and capacity. Alternatively, partially embedded black phosphorus forms a multi-layered, overlapping composite coating layer with carbon material in a flake-like form. In this case, the black phosphorus flakes have a larger diameter, such as greater than 10 nm. These large-diameter flakes can be individually coated on the surface of the core lithium manganese iron phosphate particles, or co-coated with carbon material to form a multi-layered or mixed coating layer. Specifically, the number of layers in the composite coating layer can be 1 to 10. In this embodiment, the physical doping of carbon material and black phosphorus in the coating layer forms an interlocking and overlapping composite structure. This not only combines the high conductivity and high ion / electron transport efficiency of both carbon material and black phosphorus sheets, but also reduces the impact of black phosphorus volume expansion and shortens the ion diffusion path through the doping of carbon material and black phosphorus, thereby improving the conductivity and carrier transport efficiency of the coating layer. This enhances the cycle stability and rate performance of the composite material.

[0048] In some possible implementations, nano-black phosphorus sheets include: small-sized sheets with an average thickness of 1–5 nm and an average length / width of 5–15 nm, and large-sized sheets with an average thickness of 5–15 nm and an average length / width of 15–500 nm. Small-sized black phosphorus sheets are advantageous for forming an interlocking structure with carbon materials, while large-sized sheets are advantageous for forming a multilayered overlapping structure with carbon materials. Furthermore, the nanosheet structure of black phosphorus, with its nanoscale dimensions, not only shortens the ion diffusion path and improves ion migration efficiency but also reduces the impact of volume expansion, thereby increasing the conductivity of black phosphorus.

[0049] In some possible implementations, within the coating layer, some black phosphorus replaces a portion of the carbon atoms in the carbon material as atomic clusters or single atoms, forming phosphorus-doped carbon material, thus achieving chemical doping. In this case, black phosphorus chemically dopes the carbon material, with phosphorus atoms replacing some carbon atoms, creating defects in the carbon material and providing more active sites for lithium-ion and electron transport. Simultaneously, the phosphorus atom recombination puts the carbon film material in an electron-rich state, causing the electron cloud to shift towards the bulk material and coupling, increasing the electronic conductivity of the coating layer material. Therefore, the chemical doping of carbon material and black phosphorus in the coating layer better improves the electronic conductivity and ion migration efficiency of the coating layer, thereby improving the rate performance and cycle stability of the composite material.

[0050] In some possible implementations, the mass percentage of black phosphorus in the lithium-intercalated phosphate composite material is 0.1–3 wt%. In this case, the sufficient content of black phosphorus in the lithium-intercalated phosphate composite material ensures improved conductivity and carrier transport efficiency of the coating layer, thereby contributing to improved rate performance and cycle stability of the composite material. In some specific embodiments, the mass percentage of black phosphorus in the lithium-intercalated phosphate composite material can be 0.1–0.5 wt%, 0.5–1 wt%, 1–1.5 wt%, 1.5–2 wt%, 2–2.5 wt%, 2.5–3 wt%, etc.

[0051] In some possible implementations, the coating layer in the lithium-intercalated phosphate composite material has a mass percentage content of 1 to 5 wt%; in this case, the mass percentage content of the coating layer sufficiently ensures that the coating layer improves the cycle stability and rate performance of the core lithium manganese iron phosphate.

[0052] In some possible implementations, the mass percentage of black phosphorus in the coating layer is 10–60 wt%. In this case, the doping mass content of black phosphorus in the coating layer is beneficial to improving the cycle stability, rate performance, capacity and other properties of the composite material, while avoiding the volume expansion caused by excessive black phosphorus doping.

[0053] In some possible implementations, the primary particle size of the lithium-intercalated phosphate composite material is 30–300 nm, and the secondary particle size is 0.3–10 μm. The primary particle size refers to the size of a single particle in the lithium-intercalated phosphate composite material under microscopic testing conditions; that is, the size of a single microparticle. The primary particle size is nanoscale, small in size, and has a large active specific surface area. When applied to electrode materials for secondary batteries, it is beneficial to improve ion intercalation / deintercalation efficiency, thereby improving the rate performance and cycle performance of the secondary battery. The secondary particle size refers to the size of the aggregate particles formed by stacking individual lithium-intercalated phosphate composite materials; that is, the macroscopic particle size formed by stacking multiple primary particles. The small secondary macroscopic particle size and high uniformity of the lithium-intercalated phosphate composite material are beneficial to improving the compaction density of the composite material, and its application to electrode materials for secondary batteries is conducive to improving energy density.

[0054] In some possible implementations, the coating layer thickness in the lithium-intercalated phosphate composite material is 0.5–8 nm. In this case, the coating layer of this thickness effectively improves the structural stability, cycle stability, and conductivity of the core lithium manganese iron phosphate, while also shortening the ion diffusion path and improving ion migration efficiency. This enhances the cycle stability, rate performance, and capacity of the lithium-intercalated phosphate composite material. In some specific embodiments, the coating layer thickness in the lithium-intercalated phosphate composite material can be 0.5–1 nm, 1–2 nm, 2–3 nm, 3–5 nm, 5–6 nm, 6–7 nm, 7–8 nm, etc.

[0055] In some possible implementations, the lithium-intercalated phosphate includes at least one of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, and lithium vanadium phosphate.

[0056] In some possible implementations, the lithium-intercalated phosphate in the core is lithium manganese iron phosphate, with the chemical formula LiMn. x Fe 1-x The value of PO4,x ranges from 0.2 to 0.8. Under these conditions, lithium manganese iron phosphate exhibits better safety performance, cycle performance, and characteristics such as redox potential and energy density.

[0057] The lithium-intercalated phosphate composite material provided in this application can be prepared by the following methods.

[0058] As attached Figure 1 As shown, a second aspect of this application provides a method for preparing a lithium-intercalated phosphate composite material, comprising the following steps:

[0059] S10. After obtaining each raw material component according to the stoichiometric ratio of the elements in the lithium intercalated phosphate, the raw material components are mixed and ground with red phosphorus and organic grinding aid in an inert atmosphere to obtain a mixed precursor.

[0060] S20. The mixed precursor is sintered in an inert atmosphere to obtain a core-shell structured lithium-intercalated phosphate composite material, wherein the core is a lithium-intercalated phosphate and the coating layer includes carbon material and black phosphorus forming an intercalated and overlapping composite structure.

[0061] The second aspect of this application provides a method for preparing a lithium-intercalated phosphate composite material. After obtaining the lithium-intercalated phosphate raw material components, it is mixed and ground with red phosphorus and an organic grinding aid in an inert atmosphere. This mixing and grinding process ensures thorough and uniform mixing of the raw material components. Furthermore, the localized high temperature and pressure during the mixing and grinding process initially converts the red phosphorus into black phosphorus. With the assistance of the organic grinding aid, the black phosphorus is exfoliated to produce two-dimensional black phosphorus nanosheets or atomic clusters. The inert atmosphere is used to prevent the unstable red phosphorus from being oxidized during the mixing and grinding process. The mixed precursor is then sintered in an inert atmosphere. Some of the red phosphorus remaining from the ball milling process is completely converted into black phosphorus during sintering, while the organic grinding aid is converted into carbon material during sintering. This carbon material forms an embedded and overlapping composite structure with the black phosphorus, jointly coating the surface of the phosphate particles to obtain a core-shell structured lithium-intercalated phosphate composite material. The prepared composite material exhibits higher capacity, conductivity, and rate performance, among other electrochemical properties. Moreover, this preparation process is simple, low-cost, and has minimal environmental impact, providing an optional method for the industrial preparation of high-rate-performance phosphate electrode materials.

[0062] In some possible implementations, the conditions for the mixed grinding process in step S10 include: ball milling for 6 to 24 hours in an inert atmosphere with a ball-to-material ratio of (15-50):1 and a rotation speed of 500-1200 rpm. Under these mixed grinding conditions, the uniform mixing of the raw material components is ensured, and the continuous collision and friction between the organic grinding aid and red phosphorus during the mixed grinding process generates local high temperature and high pressure, which initially transforms the red phosphorus into black phosphorus, reduces the size of the black phosphorus, forms nanosheets, shortens the ion diffusion path, reduces the impact of black phosphorus volume expansion, and improves the conductivity of black phosphorus. In some specific embodiments, the inert atmosphere includes, but is not limited to, nitrogen, argon, and helium.

[0063] Among some possible implementations, hybrid grinding processes include planetary ball milling or oscillating ball milling.

[0064] In some possible implementations, the mass percentage of red phosphorus in the mixed precursor is 0.1–3 wt%. In this case, this amount of red phosphorus sufficiently ensures the improvement of the rate performance and cycle stability of the subsequently generated black phosphorus in the lithium-intercalated phosphate composite material. If the amount added is too low, the performance improvement of the composite material will not be significant. In some specific embodiments, the mass percentage of red phosphorus in the mixed precursor can be 0.1–0.5 wt%, 0.5–1 wt%, 1–1.5 wt%, 1.5–2 wt%, 2–2.5 wt%, 2.5–3 wt%, etc.

[0065] In some possible implementations, the organic grinding aid in the mixed precursor contains 0.5–6 wt% by mass. In this case, the amount of organic grinding aid added ensures that red phosphorus is converted into nanoscale black phosphorus during the mixing and grinding process, and also ensures that the carbon material formed by the organic grinding aid after subsequent sintering is sufficient to form a composite coating layer of carbon material and black phosphorus on the surface of lithium manganese iron phosphate, thereby improving the capacity, cycle stability, and rate performance of the composite material. In some specific embodiments, the mass percentage of the organic grinding aid in the mixed precursor can be 0.5–1 wt%, 1–2 wt%, 2–3 wt%, 3–4 wt%, 4–5 wt%, 5–6 wt%, etc.

[0066] In some possible implementations, organic grinding aids include at least one of urea, starch, glucose, sucrose, fructose, and citric acid. These organic grinding aids can not only convert red phosphorus into black phosphorus with nanoscale size during the mixing and grinding process, but also transform into carbon materials after sintering, forming an embedded and overlapping composite structure with black phosphorus to jointly coat the surface of lithium manganese iron phosphate particles.

[0067] In some possible implementations, the lithium-intercalated phosphate specifically includes at least one of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, and lithium vanadium phosphate. In some specific embodiments, the chemical formula of lithium manganese iron phosphate is LiMn. x Fe 1-x The value of PO4,x ranges from 0.2 to 0.8. Under these conditions, lithium manganese iron phosphate exhibits better safety performance, cycle performance, and characteristics such as redox potential and energy density.

[0068] In some possible implementations, the raw material components obtained by lithium intercalation phosphate include lithium sources, manganese sources, iron sources, cobalt sources, vanadium sources, and phosphates. Specifically, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium acetate, and lithium oxalate. The phosphate includes at least one of phosphoric acid, ammonium dihydrogen phosphate, and ammonium phosphate. The manganese source includes at least one of manganese monoxide, manganese tetroxide, manganese trioxide, manganese acetate, manganese sulfate, manganese nitrate, and manganese chloride. The cobalt source includes at least one of cobalt sulfate, cobalt carbonate, cobalt nitrate, cobalt chloride, and cobalt acetate. The vanadium source includes at least one of vanadium acetylacetonate, vanadium dicerocene chloride, vanadium acetylacetonate, bismuth vanadate, magnesium vanadate, sodium metavanadate, ammonium metavanadate, potassium metavanadate, silver metavanadate, and sodium orthovanadate. The iron source includes at least one of ferrous sulfate, ferrous nitrate, ferrous acetate, and ferrous oxalate. The raw material components selected in the above embodiments of this application, such as lithium source, manganese source, iron source, vanadium source, cobalt source and phosphate, are easy to obtain. After mixing, grinding and sintering, lithium manganese iron phosphate material can be generated.

[0069] In some possible implementations, step S20 above includes a sintering process comprising: performing a first sintering process on the mixed precursor, followed by cooling and then a second sintering process; wherein the temperature of the first sintering process is lower than the temperature of the second sintering process. In this case, the first sintering process is first performed at a lower temperature, causing the raw material components such as lithium source, manganese source, iron source, and phosphate in the precursor to be initially converted into lithium manganese iron phosphate crystals. Simultaneously, the sintering process further converts incompletely converted red phosphorus into black phosphorus. The organic grinding aid is converted into carbon material, forming an interlocking composite structure with the black phosphorus to jointly coat the surface of the lithium manganese iron phosphate particles. Then, the second sintering process is performed at a higher temperature, further improving the crystallinity of the lithium manganese iron phosphate material and increasing the graphitization degree of the carbon material in the coating layer. This further optimizes the particle crystallinity, morphology, size, impurities, and surface coating of the composite material. This embodiment of the application employs a first sintering process at a relatively low temperature, followed by a second sintering process at a relatively high temperature after cooling. These two sintering stages at different temperatures help improve the crystallinity of the composite material particles, preventing excessive particle size and agglomeration. Simultaneously, during the sintering process, some phosphorus atoms in the black phosphorus volatilize, replacing some carbon atoms in the carbon material and forming partially phosphorus-doped carbon material. This introduces defects into the carbon material, providing more active sites for lithium ions and electron transport. Furthermore, the phosphorus atom recombination puts the carbon material in an electron-rich state, causing the electron cloud to shift towards the bulk material and coupling, thus increasing the material's electronic conductivity.

[0070] In some possible implementations, the conditions for the first sintering treatment include: heating to 200–500°C at a rate of 1–5°C / min under an inert atmosphere and holding at that temperature for 3–15 hours. Under these conditions, the raw material components such as lithium, manganese, iron, and phosphate in the precursor can be initially sintered and transformed into lithium manganese iron phosphate crystals. Simultaneously, incompletely converted red phosphorus is further converted into black phosphorus during the sintering process. Organic grinding aids are converted into carbon materials, forming an embedded and overlapping composite structure with the black phosphorus to jointly coat the surface of the lithium manganese iron phosphate particles.

[0071] In some possible implementations, the conditions for the second sintering treatment include: heating to 500–800°C at a rate of 1–5°C / min under an inert atmosphere and holding at that temperature for 3–15 hours. Under these conditions, the crystallinity of lithium manganese iron phosphate material can be further improved, as can the graphitization degree of carbon materials in the coating layer. Furthermore, the particle crystallinity, morphology, size, impurity phases, and surface coating of the composite material can be further optimized.

[0072] In some specific embodiments, the inert atmosphere includes, but is not limited to, nitrogen, argon, helium, etc.

[0073] In some possible implementations, the mass percentage of black phosphorus in the lithium-intercalated phosphate composite material is 0.1–3 wt%.

[0074] In some possible implementations, the coating layer in the lithium-intercalated phosphate composite material has a mass percentage content of 1–5 wt%.

[0075] In some possible implementations, the coating layer contains 10–60 wt% black phosphorus.

[0076] In some possible implementations, the primary particle size of the lithium-intercalated phosphate composite material is 30–300 nm, and the secondary particle size is 0.3–10 μm.

[0077] In some possible implementations, the coating thickness in the lithium-intercalated phosphate composite material is 0.5–8 nm.

[0078] In some possible implementations, in the coating layer, some black phosphorus replaces some carbon atoms in the carbon material in the form of atomic clusters or single atoms to form phosphorus-doped carbon material.

[0079] In some possible implementations, the embedded and overlapping composite structure includes: a portion of black phosphorus is embedded in carbon material in the form of nano-black phosphorus sheets to form a composite coating layer; a portion of black phosphorus is in the form of sheets to form a multi-layered overlapping composite coating layer with carbon material.

[0080] In some possible implementations, the nano-black phosphorus sheets in the coating layer of the lithium-intercalated phosphate composite material include: small-sized sheets with an average thickness of 1-5 nm and an average length / width of 5-15 nm, and large-sized sheets with an average thickness of 5-15 nm and an average length / width of 15-1000 nm.

[0081] The beneficial effects of the above-described embodiments of the lithium-intercalated phosphate composite material have been discussed in detail above and will not be repeated here.

[0082] A third 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 contains the above-mentioned lithium-intercalated phosphate composite material or the lithium-intercalated phosphate composite material prepared by the above method.

[0083] The secondary battery provided in the third aspect of this application includes a positive electrode comprising the aforementioned lithium-intercalated phosphate composite material. This lithium-intercalated phosphate composite material has a core-shell structure, with a core of lithium manganese iron phosphate and a coating layer comprising carbon material and black phosphorus forming an intercalated and overlapping composite structure. It possesses characteristics such as high capacity, high rate performance, and high cycle stability. Therefore, it can improve the energy density, cycle stability, and rate performance of the secondary battery.

[0084] 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.

[0085] 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).

[0086] In some possible implementations, the electrolyte is an organic solution containing soluble lithium salts.

[0087] To enable those skilled in the art to clearly understand the above-described implementation details and operations, and to demonstrate the significant improvement in the performance of the lithium-intercalated phosphate composite material and its preparation method, as well as the secondary battery, the following examples illustrate the above technical solutions.

[0088] Example 1

[0089] A composite material of lithium manganese iron phosphate co-coated with black phosphorus and carbon, the preparation of which includes the following steps:

[0090] ① Weigh 3.69g (0.05mol) lithium carbonate, 11.50g (0.1mol) ammonium dihydrogen phosphate, 5.34g (~0.0233mol) manganese tetroxide, and 5.38g (~0.003mol) ferrous oxalate dihydrate into a ball mill jar. Then weigh 0.259g (0.1wt%) red phosphorus, 0.259g urea, and 0.13g glucose into the ball mill jar. Add 380g of ball milling beads of different sizes and ball mill at 700rpm for 8h under a nitrogen protective atmosphere to obtain a mixed precursor.

[0091] ② The mixed precursor obtained by ball milling was calcined in a tube furnace at 350℃ for 8 hours under a nitrogen atmosphere. The heating rate was set to 2℃ / min. After natural cooling, a primary sintering product was obtained.

[0092] ② The above-mentioned primary sintering product was further calcined in a tube furnace at 600℃ for 8 hours under a nitrogen atmosphere, with a heating rate of 2℃ / min. After natural cooling, a composite material of lithium manganese iron phosphate co-coated with black phosphorus and carbon was obtained, denoted as LiMn. 0.7 Fe 0.3 PO4@BP@C-1.

[0093] Example 2

[0094] A composite material of lithium manganese iron phosphate co-coated with black phosphorus and carbon, the preparation of which includes the following steps:

[0095] ① Weigh 2.39g (0.1mol) of lithium hydroxide (LiOH), 11.50g (0.1mol) of ammonium dihydrogen phosphate (NH4H2PO4), 12.11g (0.07mol) of manganese acetate ((CH3COO)2Mn), and 8.34g (0.03mol) of ferrous sulfate heptahydrate (FeSO4·7H2O) into a ball mill jar. Then weigh 2.59g (1wt%) of red phosphorus, 0.15g of glucose, and 0.15g of starch into the ball mill jar. Add 500g of ball milling beads of different sizes and ball mill at 800rpm for 15h under a nitrogen protective atmosphere to obtain a mixed precursor.

[0096] ② The mixed precursor obtained by ball milling was calcined in a tube furnace at 400℃ for 12h under a nitrogen atmosphere, with a heating rate of 2℃ / min. After natural cooling, a primary sintering product was obtained.

[0097] ② The above-mentioned primary sintering product was further calcined in a tube furnace at 650℃ for 8 hours under a nitrogen atmosphere, with a heating rate of 5℃ / min. After natural cooling, a composite material of lithium manganese iron phosphate co-coated with black phosphorus and carbon was obtained, denoted as LiMn. 0.7 Fe 0.3 PO4@BP@C-2.

[0098] Example 3

[0099] A composite material of lithium manganese iron phosphate co-coated with black phosphorus and carbon, the preparation of which includes the following steps:

[0100] ① Weigh 6.599g (0.1mol) of lithium acetate (CH3COOLi), 14.91g (0.1mol) of ammonium phosphate ((NH4)3PO4), 4.96g (0.07mol) of manganese monoxide (MnO), and 5.396g (0.03mol) of ferrous nitrate (Fe(NO3)2) into a ball mill jar. Then weigh 0.6373g (2wt%) of red phosphorus, 0.3187g of sucrose, and 0.3g of citric acid into the ball mill jar. Add 600g of ball milling beads of different sizes and ball mill at 1000rpm for 18h under a nitrogen protective atmosphere to obtain a mixed precursor.

[0101] ② The mixed precursor obtained by ball milling was calcined in a tube furnace at 500℃ for 6 hours under a nitrogen atmosphere. The heating rate was set to 5℃ / min. After natural cooling, a primary sintering product was obtained.

[0102] ② The above-mentioned primary sintering product was further calcined in a tube furnace at 750℃ for 8 hours under a nitrogen atmosphere, with a heating rate of 5℃ / min. After natural cooling, a lithium manganese iron phosphate composite material co-coated with black phosphorus and carbon was obtained, denoted as LiMn. 0.7 Fe 0.3 PO4@BP@C-3.

[0103] Comparative Example 1

[0104] A carbon-coated lithium manganese iron phosphate composite material is prepared in a manner different from that in Example 1, where red phosphorus is not added in step ①, ultimately yielding a carbon-coated lithium manganese iron phosphate composite material LiMn. 0.7 Fe 0.3 PO4@C.

[0105] Comparative Example 2

[0106] A black phosphorus and carbon co-coated lithium manganese iron phosphate composite material is prepared in a manner different from that in Example 1, urea and glucose are not added in step ①. The final product is a black phosphorus and carbon co-coated lithium manganese iron phosphate composite material, LiMn, with extremely low carbon content. 0.7 Fe 0.3 PO4@BP@C.

[0107] Comparative Example 3

[0108] A black phosphorus-coated lithium manganese iron phosphate composite material was prepared in a manner different from that in Example 1, the ball milling beads used in step ① weighed 150g and rotated at 300 rpm. The final product was a black phosphorus and carbon co-coated lithium manganese iron phosphate composite material (LiMn) containing red phosphorus impurities and not exfoliated into nanosheets. 0.7 Fe 0.3 PO4@BP@C.

[0109] Furthermore, to verify the progressiveness of the embodiments of this application, the following performance tests were performed on the embodiments and comparative examples respectively:

[0110] 1. The black phosphorus and carbon co-coated lithium manganese iron phosphate composite material (LiMn) prepared in Example 1 0.7 Fe 0.3 X-ray powder diffraction tests were performed on PO4@BP@C-1, and the results are attached. Figure 2 As shown in the attached figures, the X-ray diffraction pattern of the black phosphorus and carbon co-coated lithium manganese iron phosphate composite material prepared in Example 1 is basically consistent with the standard card PDF-#13-0336.

[0111] 2. The black phosphorus and carbon co-coated lithium manganese iron phosphate composite material (LiMn) prepared in Example 1 0.7 Fe 0.3 Morphological observations were performed on PO4@BP@C-1, and the SEM images are attached. Figure 3 As shown, the composite material particles are uniform and have a high uniformity in particle size.

[0112] 3. Battery performance testing: Using the lithium manganese iron phosphate composite material provided in the above examples and comparative examples, the battery was assembled according to the following method:

[0113] ① Preparation of positive electrode sheet: The lithium manganese iron phosphate composite materials prepared in each example and comparative example were mixed with SP (conductive carbon black), PVDF (polyvinylidene fluoride) and NMP (N-methylpyrrolidone) in a mass ratio of 93.5:2.5:4:100 and stirred in a ball mill for 2 hours to obtain a uniform positive electrode slurry. The prepared positive electrode slurry was added to aluminum foil, evenly scraped with a scraper, dried at 130°C and then rolled to obtain a positive electrode sheet.

[0114] ② Battery assembly process: The prepared positive electrode is attached to the positive electrode metal shell with conductive adhesive, a lithium metal sheet is used as the negative electrode, a Celgard 2400 microporous membrane is used as the separator, and a 1.0 mol / L LiPF6 solution is used as the electrolyte. The solvent of the electrolyte is a mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) in a volume ratio of 1:1:1. The cells are assembled into a button cell in a glove box.

[0115] Using a LAND electrochemical tester, the 0.1C, 1C, 5C, and 10C discharge capacities of the aforementioned coin cells were tested under the conditions of a charging termination voltage of 4.2V and a discharging termination voltage of 2.0V. The capacity retention and resistivity after 200 charge-discharge cycles at 1C were also tested. The test results are shown in Table 1 below.

[0116] Table 1

[0117]

[0118] Among them, the black phosphorus and carbon co-coated lithium manganese iron phosphate composite material (LiMn) prepared in Example 1 0.7 Fe 0.3 The carbon-coated lithium manganese iron phosphate composite material (LiMn) prepared by PO4@BP@C-1 and Comparative Example 1 was also tested. 0.7 Fe 0.3 The capacity-voltage curves of coin cells prepared by PO4@C at a 0.1C rate are shown in the attached figure. Figure 4 As shown in the attached figure, the specific capacity of coin cells made from the lithium manganese iron phosphate composite materials prepared in Example 1 and Comparative Examples 1 and 2 at different rates is shown in the attached figure. Figure 5 As shown.

[0119] The test results above show that, compared with the lithium manganese iron phosphate composite material with only carbon coating in Comparative Example 1, the lithium manganese iron phosphate composite material with black phosphorus and carbon coating containing very little carbon in Comparative Example 2, and the lithium manganese iron phosphate composite material with black phosphorus and carbon coating containing red phosphorus impurities and not peeled into nanosheets in Comparative Example 3, the lithium manganese iron phosphate composite material with black phosphorus and carbon co-coated by red phosphorus peeling into black phosphorus and carbon materials to form physical and chemical doping has better electrical conductivity. After being made into a secondary battery, the battery exhibits better rate performance and cycle stability.

[0120] 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 lithium-intercalated phosphate composite material, characterized in that, The system comprises a lithium-intercalated phosphate core and a coating layer covering the outer surface of the core; the coating layer comprises carbon material and black phosphorus, wherein the black phosphorus and the carbon material form an intercalated and overlapping composite structure; the black phosphorus comprises physical doping with the carbon material in the form of nanosheets, and chemical doping with the carbon material in the form of atomic clusters or individual atoms; the coating layer is formed by a method comprising the following steps: In an inert atmosphere, the raw material components of the lithium-intercalated phosphate are mixed and ground with red phosphorus and organic grinding aid to initially convert the red phosphorus into black phosphorus. With the assistance of the organic grinding aid, the black phosphorus is exfoliated to produce two-dimensional black phosphorus nanosheets or atomic clusters, thus obtaining a mixed precursor. The mixed precursor is sintered in an inert atmosphere to transform the raw material components into the core of the lithium-intercalated phosphate, and the red phosphorus remaining from part of the ball milling process is completely transformed into black phosphorus. The organic grinding aid is transformed into carbon material, thereby forming the coating layer on the outer surface of the core of the lithium-intercalated phosphate.

2. The lithium-intercalated phosphate composite material as described in claim 1, characterized in that, The inlaid and overlapping composite structure includes: a portion of the black phosphorus is embedded in the carbon material in the form of nano-black phosphorus sheets to form a composite coating layer; a portion of the black phosphorus is in the form of sheets to form a multi-layered overlapping composite coating layer with the carbon material; And / or, in the coating layer, a portion of the black phosphorus replaces some of the carbon atoms in the carbon material in the form of atomic clusters or single atoms to form a phosphorus-doped carbon material; And / or, the lithium intercalated phosphate includes at least one of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, and lithium vanadium phosphate.

3. The lithium-intercalated phosphate composite material as described in claim 2, characterized in that, In the lithium-intercalated phosphate composite material, the black phosphorus content is 0.1~3 wt% by mass. And / or, in the lithium-intercalated phosphate composite material, the mass percentage content of the coating layer is 1~5 wt%; And / or, in the coating layer, the mass percentage of black phosphorus is 10~60 wt%; And / or, the nano-black phosphorus sheet includes: a small-sized sheet with an average thickness of 1~5 nm and an average length / width of 5~15 nm, and a large-sized sheet with an average thickness of 5~15 nm and an average length / width of 15~1000 nm.

4. The lithium-intercalated phosphate composite material according to any one of claims 1 to 3, characterized in that, The primary particle size of the lithium-intercalated phosphate composite material is 30~500 nm, and the secondary particle size is 0.3~10 μm; And / or, the coating layer thickness in the lithium-intercalated phosphate composite material is 0.5~8 nm.

5. A method for preparing a lithium-intercalated phosphate composite material as described in any one of claims 1 to 4, characterized in that, Includes the following steps: According to the stoichiometric ratio of the elements in the lithium intercalated phosphate, the raw material components are obtained and then mixed and ground with red phosphorus and organic grinding aid in an inert atmosphere to obtain a mixed precursor. The mixed precursor is sintered in an inert atmosphere to obtain a core-shell structured lithium-intercalated phosphate composite material, wherein the core is a lithium-intercalated phosphate and the coating layer includes carbon material and black phosphorus forming an intercalated and overlapping composite structure.

6. The method for preparing the lithium-intercalated phosphate composite material as described in claim 5, characterized in that, The sintering process includes: performing a first sintering process on the mixed precursor, cooling it, and then performing a second sintering process; wherein the temperature of the first sintering process is lower than the temperature of the second sintering process. And / or, the conditions for the mixed grinding treatment include: ball milling for 6 to 24 hours in an inert atmosphere with a ball-to-material ratio of (15~50):1 and a rotation speed of 500~1200 rpm.

7. The method for preparing the lithium-intercalated phosphate composite material as described in claim 6, characterized in that, The conditions for the first sintering treatment include: heating to 200-500 °C at a rate of 1-5 °C / min under an inert atmosphere and holding at that temperature for 3-15 h; And / or, the conditions for the second sintering treatment include: heating to 500-800 °C at a rate of 1-5 °C / min under an inert atmosphere and holding at that temperature for 3-15 h; And / or, the mixed grinding process includes planetary ball milling or oscillating ball milling.

8. The method for preparing the lithium-intercalated phosphate composite material according to any one of claims 5 to 7, characterized in that, In the mixed precursor, the red phosphorus has a mass percentage content of 0.1~3 wt%; And / or, in the mixed precursor, the organic grinding aid has a mass percentage content of 0.5~6wt%.

9. The method for preparing the lithium-intercalated phosphate composite material as described in claim 8, characterized in that, The organic grinding aid includes at least one of urea, starch, glucose, sucrose, fructose, and citric acid.

10. 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 contains a lithium-intercalated phosphate composite material as described in any one of claims 1 to 4 or a lithium-intercalated phosphate composite material prepared by the method described in any one of claims 5 to 9.

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