Flaky composite lithium manganese iron phosphate material and preparation method thereof

Abstract

The application provides a sheet-shaped composite manganese iron lithium phosphate material and a preparation method thereof. The sheet-shaped composite manganese iron lithium phosphate material comprises a composite manganese iron lithium phosphate core and a carbon coating layer coated on the surface of the composite manganese iron lithium phosphate core, wherein the chemical formula of the composite manganese iron lithium phosphate core is LiFe x Mn y A z PO4, wherein 0.2≤x≤0.4, 0.57≤y≤0.78, and x+y+z=1; A is Mg element, V element and Nb element; the sheet diameter and the tap density of the sheet-shaped composite manganese iron lithium phosphate material are 0.5-2 μm and 0.9-1.2 g / cc, respectively. The application dopes Mg element, V element and Nb element and controls the sheet diameter of the sheet-shaped composite manganese iron lithium phosphate material to be 0.5-2 μm, so that the energy density and the powder bulk density of the sheet-shaped composite manganese iron lithium phosphate material are improved compared with the original morphology of manganese iron lithium phosphate.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a sheet-like composite lithium manganese iron phosphate material and its preparation method. Background Technology

[0002] Lithium manganese iron phosphate (LMFP), as an "upgraded version" of lithium iron phosphate (LFP) cathode materials, inherits the advantages of LFP such as low cost, high thermal stability, and high safety, while overcoming its disadvantages such as low energy density and poor low-temperature performance. However, LMFP also suffers from poor conductivity, rate performance, and cycle performance. Currently, coating, nano-sizing, and morphology control shorten the migration path of lithium ions and construct a fast conductive network by altering external characteristics; ion doping changes the internal crystal structure, creating defects in the original lattice, thus enhancing the conductivity of Li-ion ions. + Expanding diffusion channels and increasing the carrier density of the material can both improve the electrochemical performance of LMFPs to some extent.

[0003] In the prior art, the preparation method of lithium manganese iron phosphate, such as Chinese patent application No. 202210309750.1, provides a single-core multi-shell lithium manganese iron phosphate cathode material. It prepares carbon-coated lithium manganese iron phosphate particles of different particle sizes; using the carbon-coated lithium iron phosphate particles as the core, and preparing lithium manganese iron phosphate coating layers on the outer surface of the core in order of increasing particle size, forming multiple lithium manganese iron phosphate coating layers, and obtaining a single-core multi-shell lithium manganese iron phosphate composite material. However, the multi-layer coating results in a complex process, which is not conducive to large-scale production. For example, Chinese patent application No. 202111667352.9 provides a high-compaction phosphate-type cathode material, which includes a dual-morphology phosphate-type cathode material. This dual-morphology phosphate-type cathode material includes spherical phosphate-type cathode material and sheet-like phosphate-type cathode material. By compounding the spherical phosphate-type cathode material and the sheet-like phosphate-type cathode material, the compaction density and capacity ratio of the cathode material are taken into account, thereby synergistically improving the energy density of the cathode material. However, the simple mixing of dual-morphology phosphate-type cathode materials may have problems such as poor uniformity and loose local particle contact, which may lead to the material performance not being fully utilized.

[0004] Although the size of common lithium manganese iron phosphate particles (between 100nm and 200nm) is smaller than that of lithium iron phosphate (300nm to 400nm), smaller particles of lithium manganese iron phosphate can bring higher capacity and better rate capability, but they also cause problems such as lower compaction density and processing difficulties.

[0005] Currently, the energy density of lithium manganese iron phosphate (LMP) materials is still lower than that of mature lithium iron phosphate (LFP) materials. This is mainly because LMP materials cannot simultaneously achieve higher compaction density and specific capacity than LFP materials. For LMP materials, methods that improve the material's conductivity and lithium-ion diffusion rate often reduce the compaction density, thus making it difficult to effectively increase the energy density. Summary of the Invention

[0006] The main objective of this invention is to provide a sheet-like composite lithium manganese iron phosphate material and its preparation method, so as to solve the problem of low energy density of lithium manganese iron phosphate materials in the prior art.

[0007] To achieve the above objectives, according to one aspect of the present invention, a sheet-like composite lithium manganese iron phosphate material is provided, comprising a composite lithium manganese iron phosphate core and a carbon coating layer covering the surface of the composite lithium manganese iron phosphate core, wherein the chemical formula of the composite lithium manganese iron phosphate core is LiFe. x Mn y A z PO4, wherein 0.2≤x≤0.4, 0.57≤y≤0.78, x+y+z=1; A represents Mg, V and Nb elements; the sheet diameter of the sheet-like composite lithium manganese iron phosphate material is 0.5~2μm, and the tap density of the sheet-like composite lithium manganese iron phosphate material is 0.9g / cc~1.2g / cc.

[0008] Furthermore, the lattice parameters of the above-mentioned sheet-like composite lithium manganese iron phosphate material are as follows:

[0009] The sheet-like structure of the sheet-like composite lithium manganese iron phosphate material in this application helps to reduce the size of the sheet-like composite lithium manganese iron phosphate material in the b-axis direction and expand its size in the a-axis and c-axis directions, thereby greatly shortening the lithium ion diffusion path and having better lithium ion diffusion kinetics performance. At the same time, it takes into account the compaction density and capacity ratio of the sheet-like composite lithium manganese iron phosphate material, thus synergistically improving the energy density of the sheet-like composite lithium manganese iron phosphate material.

[0010] Furthermore, the mass content of Mg is 2000ppm to 3000ppm, the mass content of V is 100ppm to 1000ppm, and the mass content of Nb is 1000ppm to 2000ppm.

[0011] By controlling the mass content ranges of doping elements Mg, Nb, and V, as well as the ratio of their mass contents within the aforementioned ranges, it is helpful to enhance the synergistic effect of the three doping elements, thereby synergistically improving the energy density of the sheet-like composite lithium manganese iron phosphate material.

[0012] Furthermore, the thickness of the above-mentioned sheet-like composite lithium manganese iron phosphate material is 0.2 μm to 0.5 μm; and / or, the mass of the carbon coating layer is 1.5 to 2% of the mass of the sheet-like composite lithium manganese iron phosphate material.

[0013] By controlling the thickness of the sheet-like composite lithium manganese iron phosphate material within the aforementioned range, it is easier to improve the dimensional matching between the sheet-like structures, thereby increasing the packing density and reducing the lithium-ion insertion / extraction pathways. This, in turn, is beneficial to improving the electrochemical performance, especially the rate performance, of the sheet-like composite lithium manganese iron phosphate material. The preferred carbon coating content (as described above) helps to fully utilize the activity of manganese ions, thereby improving the electrical performance of the sheet-like composite lithium manganese iron phosphate material.

[0014] According to another aspect of the present invention, a method for preparing the aforementioned sheet-like composite lithium manganese iron phosphate material is provided. The method includes: step S1, mixing raw materials including lithium source, iron source, manganese source, phosphorus source, carbon source, and dopant source and then grinding them to obtain a ground slurry; step S2, spraying the ground slurry to obtain a yellow material, and sintering the yellow material in a protective atmosphere to obtain a black material; step S3, activating the raw materials including the black material, molten salt medium, and conductive additives by high-energy ball milling to obtain an activated mixture; step S4, sequentially melting, filtering, washing, and drying the activated mixture to obtain the sheet-like composite lithium manganese iron phosphate material; wherein the dopant source includes Mg source, V source, and Nb source.

[0015] On the one hand, this application activates the molten salt medium through high-energy ball milling, resulting in a liquid phase during the synthesis of the composite lithium manganese iron phosphate material. This allows the reactants to have a certain solubility within the liquid phase, transforming the reaction from a solid-solid reaction to a solid-liquid reaction. This significantly accelerates the diffusion rate of reactant ions, avoiding the problems of localized inhomogeneity and high energy consumption inherent in traditional high-temperature solid-phase methods, and also overcoming the limitations of hydrothermal methods. On the other hand, the high shear force generated during the mechanical melting process of the nanoparticle raw materials in the liquid molten salt reshapes the microstructure of the nanoparticles into a sheet-like morphology. Compared to the original spherical lithium manganese iron phosphate particles, the packing density of the sheet-like composite lithium manganese iron phosphate material is increased, thereby improving the energy density of the phosphate-type cathode material. Simultaneously, this application, by introducing Mg, V, and Nb elements into the lithium manganese iron phosphate structure, combines the advantages of each doped metal ion, improving the material's crystal structure, conductivity, and cycle performance.

[0016] Further, in step S1 above, the carbon source includes glucose, preferably a mixture of glucose and polyvinyl alcohol, preferably a mass ratio of glucose to polyvinyl alcohol of (7-9):(3-1), preferably polyvinyl alcohol of the 88 series; and / or the Mg source is selected from any one or more of magnesium oxide, magnesium carbonate, magnesium acetate, magnesium lactate, magnesium dihydrogen phosphate and magnesium hydroxide; and / or the V source is selected from any one or more of vanadium pentoxide, ammonium metavanadate, vanadium carbide and vanadium oxalate; and / or the Nb source is niobium pentoxide and / or niobium oxalate; and / or the lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium acetate and lithium oxalate; and / or the iron source is selected from any one or more of iron phosphate, ferric oxide, iron tetroxide, iron hydroxide, ferric lactate and ferrous oxalate; and / or the manganese source is selected from any one or more of manganese tetroxide, manganese carbonate, manganese dioxide and manganese trioxide; and / or the phosphorus source is selected from any one or more of phosphoric acid, iron phosphate, ammonium phosphate, diammonium hydrogen phosphate and ammonium dihydrogen phosphate.

[0017] Preferred carbon sources are a mixture of glucose and polyvinyl alcohol, and controlling their mass ratio within the above range helps to utilize their different carbonization temperatures, resulting in more uniform doping in the sheet-like composite lithium manganese iron phosphate material. Preferred Mg, V, and Nb sources are those mentioned above, as they are more easily dissolved in the liquid-phase molten salt medium, thus allowing for more uniform doping within the lithium manganese iron phosphate crystal structure and optimizing the lithium manganese iron phosphate lattice. Preferred iron, phosphorus, and manganese sources help improve their synergistic effect with other metal sources.

[0018] Furthermore, in step S1 above, the raw materials also include a solvent, preferably selected from any one or more of water, ethanol, methanol and glycerol, and / or the solid content of the slurry after grinding is 30% to 50%, and / or the grinding particle size D50 during the grinding process is controlled to be 0.3 μm to 0.4 μm.

[0019] The preferred solvent is one or more of the types mentioned above, which helps to improve the uniform dispersion of raw materials during the grinding process, avoids material aggregation, and ensures the uniformity of the grinding process. The preferred solid content of the slurry after grinding is within the above range, which helps to improve the diffusion rate of lithium ions, thereby improving the energy density and performance of the composite lithium manganese iron phosphate material. By controlling the abrasive particle size during the grinding process within the above range, it helps to improve the diffusion coefficient of lithium ions, thus increasing the compaction density of the composite lithium manganese iron phosphate material while ensuring high capacity and high rate capability, thereby improving its charge-discharge performance and cycle stability.

[0020] Furthermore, in step S2 above, the protective atmosphere is a nitrogen atmosphere; and / or the sintering temperature is 700℃~750℃, and the sintering time is 6h~10h.

[0021] The above-mentioned optimized conditions are beneficial to improving the efficiency and effect of sintering, thereby improving the high crystallinity and stability of the structure and crystal form of the sheet-like composite lithium manganese iron phosphate material.

[0022] Further, in step S3 above, the high-energy ball milling speed is 400 rpm to 800 rpm; the high-energy ball milling time is 3 h to 8 h; the mass ratio of black material to molten salt medium is 1:(3 to 5), and / or the melting temperature of the molten salt medium is below 600°C, preferably the melting temperature of the molten salt medium is 170°C to 600°C, preferably the molten salt medium is selected from at least two of LiCl, Li2SO4, LiClO4, LiBO2, LiAc, LiNO3, LiOH, Li2CO3 and LiBr; and / or the conductive additive is a sheet-like conductive agent, preferably the sheet-like conductive agent is selected from any one or more of sheet graphite, graphene and sheet MXene.

[0023] Controlling the rotational speed and time of high-energy ball milling within the above ranges helps improve its efficiency and effectiveness. Preferably, the melting temperature of the molten salt medium within the above range helps provide a liquid phase medium for high-energy ball milling, thereby promoting the transformation of raw nanoparticles into sheet-like particles. This solves the current technical problem of insufficient energy density caused by the inability to simultaneously improve compaction density and specific capacity due to the small particle size of lithium manganese iron phosphate materials. Preferably controlling the mass ratio of black material to molten salt medium within the above range is beneficial for improving the dispersion effect of the black material in the liquid-phase molten salt medium. The preferred conductive additives of the above types help improve the conductivity of the sheet-like composite lithium manganese iron phosphate materials.

[0024] Furthermore, in step S4 above, the temperature during the melting process is 400℃~600℃, and the holding time is 5h~10h.

[0025] By controlling the melting temperature and holding time within the above-mentioned ranges, it is helpful to improve the efficiency and effect of melting the activated mixture, thereby improving the electrochemical performance of the sheet-like composite lithium manganese iron phosphate material.

[0026] By applying the technical solution of this invention, the sheet-like composite lithium manganese iron phosphate material of this application, through the doping of Mg, V, and Nb elements into the lithium manganese iron phosphate structure, can combine the advantages of each doped metal ion, thereby improving the crystal structure, conductivity, and cycle performance of the sheet-like composite lithium manganese iron phosphate material. Specifically, due to Mg... 2+ Ionic radius smaller than Mn 2+ The MO bond length in the doped MO6 octahedron becomes shorter, while the Li-O bond length in the LiO6 octahedron becomes longer. This extension of the Li-O bonds results in the Li... + The diffusion channels are wider, Li +Easier migration leads to increased carrier density in sheet-like composite lithium manganese iron phosphate materials, which in turn benefits the electrochemical performance of multi-component olivine-structured cathode materials. The introduction of multivalent metal ions V... 5+ and Nb 5+ Ion defects were generated, and mixed valence states favored Li + Intercalation / deintercalation within the material. This involves altering the crystal structure of lithium manganese iron phosphate (LFP) through ion doping, creating defects in the original LFP lattice, thus enabling Li… + The expansion of diffusion channels and the increase in carrier density of the sheet-like composite lithium manganese iron phosphate material can improve its electrochemical performance to some extent. Therefore, by doping with Mg, V, and Nb elements, the lithium manganese iron phosphate nanoparticles are restructured into a sheet-like morphology, resulting in a sheet diameter of 0.5 μm to 2 μm. This improves the powder packing density of the sheet-like composite lithium manganese iron phosphate material compared to the original morphology, thereby increasing its energy density. Furthermore, the carbon coating layer significantly enhances the energy density, discharge specific capacity, rate performance, and cycle stability of the sheet-like composite lithium manganese iron phosphate material. Attached Figure Description

[0027] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0028] Figure 1 The phase diagram curve of LiOH-LiCl is shown;

[0029] Figure 2 The XRD pattern of the composite lithium iron manganese phosphate electrode material prepared in Example 1 is shown;

[0030] Figure 3 SEM images of the composite lithium iron manganese phosphate electrode material prepared in Example 1 are shown.

[0031] Figure 4 The image shows a SEM image of commercially available nano-lithium iron manganese phosphate material. Detailed Implementation

[0032] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0033] As analyzed in the background section, existing lithium manganese iron phosphate materials suffer from low energy density. To address this issue, this invention provides a sheet-like composite lithium manganese iron phosphate material and its preparation method.

[0034] In a typical embodiment of this application, a sheet-like composite lithium manganese iron phosphate material is provided, comprising a composite lithium manganese iron phosphate core and a carbon coating layer covering the surface of the composite lithium manganese iron phosphate core, wherein the chemical formula of the composite lithium manganese iron phosphate core is LiFe. x Mn y A z PO4, where 0.2≤x≤0.4, 0.57≤y≤0.78, x+y+z=1; A represents Mg, V and Nb elements; the sheet diameter of the sheet-like composite lithium manganese iron phosphate material is 0.5μm~2μm, and the tap density of the sheet-like composite lithium manganese iron phosphate material is 0.9g / cc~1.2g / cc.

[0035] In the sheet-like composite lithium manganese iron phosphate material of this application, by doping the lithium manganese iron phosphate structure with Mg, V, and Nb elements, the advantages of each doped metal ion can be combined, thereby improving the crystal structure, conductivity, and cycle performance of the sheet-like composite lithium manganese iron phosphate material. Specifically, due to Mg... 2+ Ionic radius smaller than Mn 2+ The MO bond length in the doped MO6 octahedron becomes shorter, while the Li-O bond length in the LiO6 octahedron becomes longer. This extension of the Li-O bonds results in the Li... + The diffusion channels are wider, Li + Easier migration leads to increased carrier density in sheet-like composite lithium manganese iron phosphate materials, which in turn benefits the electrochemical performance of multi-component olivine-structured cathode materials. The introduction of multivalent metal ions V... 5+ and Nb 5+ Ion defects were generated, and mixed valence states favored Li + Intercalation / deintercalation within the material. This involves altering the crystal structure of lithium manganese iron phosphate (LFP) through ion doping, creating defects in the original LFP lattice, thus enabling Li… + The expansion of diffusion channels and the increase in carrier density of the sheet-like composite lithium manganese iron phosphate material can improve its electrochemical performance to some extent. Therefore, by doping with Mg, V, and Nb elements, the lithium manganese iron phosphate nanoparticles are restructured into a sheet-like morphology, resulting in a sheet diameter of 0.5 μm to 2 μm. This improves the powder packing density of the sheet-like composite lithium manganese iron phosphate material compared to the original morphology, thereby increasing its energy density. Furthermore, the carbon coating layer significantly enhances the energy density, discharge specific capacity, rate performance, and cycle stability of the sheet-like composite lithium manganese iron phosphate material.

[0036] In one embodiment of this application, the lattice parameters of the above-mentioned sheet-like composite lithium manganese iron phosphate material are as follows:

[0037] Compared with traditional spherical lithium manganese iron phosphate particles, the sheet-like structure of the sheet-like composite lithium manganese iron phosphate material in this application helps to reduce the size of the sheet-like composite lithium manganese iron phosphate material in the b-axis direction and expand its size in the a-axis and c-axis directions, thereby greatly shortening the lithium ion diffusion path and exhibiting better lithium ion diffusion kinetics performance. At the same time, it takes into account the compaction density and capacity ratio of the sheet-like composite lithium manganese iron phosphate material, thus synergistically improving the energy density of the sheet-like composite lithium manganese iron phosphate material.

[0038] In one embodiment of this application, the mass content of Mg is 2000ppm to 3000ppm, the mass content of V is 100ppm to 1000ppm, and the mass content of Nb is 1000ppm to 2000ppm.

[0039] By controlling the mass content ranges of doping elements Mg, Nb, and V, as well as the ratio of their mass contents within the aforementioned ranges, it is helpful to enhance the synergistic effect of the three doping elements, thereby synergistically improving the energy density of the sheet-like composite lithium manganese iron phosphate material.

[0040] In one embodiment of this application, the thickness of the above-mentioned sheet-like composite lithium manganese iron phosphate material is 0.2 μm to 0.5 μm; and / or, the mass of the carbon coating layer is 1.5 to 2% of the mass of the sheet-like composite lithium manganese iron phosphate material.

[0041] By controlling the thickness of the sheet-like composite lithium manganese iron phosphate material within the aforementioned range, it is easier to improve the dimensional matching between the sheet-like structures, thereby increasing the packing density and reducing the lithium-ion insertion / extraction pathways. This, in turn, is beneficial to improving the electrochemical performance, especially the rate performance, of the sheet-like composite lithium manganese iron phosphate material. The preferred carbon coating content (as described above) helps to fully utilize the activity of manganese ions, thereby improving the electrical performance of the sheet-like composite lithium manganese iron phosphate material.

[0042] In another typical embodiment of this application, a method for preparing a sheet-like composite lithium manganese iron phosphate material is provided. The method includes: step S1, mixing raw materials including lithium source, iron source, manganese source, phosphorus source, carbon source, and dopant source and then grinding them to obtain a ground slurry; step S2, spraying the ground slurry to obtain a yellow material, and sintering the yellow material in a protective atmosphere to obtain a black material; step S3, activating the raw materials including the black material, molten salt medium, and conductive additives by high-energy ball milling to obtain an activated mixture; step S4, sequentially melting, filtering, washing, and drying the activated mixture to obtain a sheet-like composite lithium manganese iron phosphate material; wherein the dopant source includes Mg source, V source, and Nb source.

[0043] On the one hand, this application activates the molten salt medium through high-energy ball milling, resulting in a liquid phase during the synthesis of the composite lithium manganese iron phosphate material. This allows the reactants to have a certain solubility within the liquid phase, transforming the reaction from a solid-solid reaction to a solid-liquid reaction. This significantly accelerates the diffusion rate of reactant ions, avoiding the problems of localized inhomogeneity and high energy consumption inherent in traditional high-temperature solid-phase methods, and also overcoming the limitations of hydrothermal methods. On the other hand, the high shear force generated during the mechanical melting process of the nanoparticle raw materials in the liquid molten salt reshapes the microstructure of the nanoparticles into a sheet-like morphology. Compared to the original spherical lithium manganese iron phosphate particles, the packing density of the sheet-like composite lithium manganese iron phosphate material is increased, thereby improving the energy density of the phosphate-type cathode material. Simultaneously, this application, by introducing Mg, V, and Nb elements into the lithium manganese iron phosphate structure, combines the advantages of each doped metal ion, improving the material's crystal structure, conductivity, and cycle performance.

[0044] In one embodiment of this application, in step S1 above, the carbon source includes glucose, preferably a mixture of glucose and polyvinyl alcohol, preferably a mass ratio of glucose to polyvinyl alcohol of (7-9):(3-1), and preferably polyvinyl alcohol of the 88 series; and / or the Mg source is selected from any one or more of magnesium oxide, magnesium carbonate, magnesium acetate, magnesium lactate, magnesium dihydrogenate and magnesium hydroxide; and / or the V source is selected from any one or more of vanadium pentoxide, ammonium metavanadate, vanadium carbide and vanadium oxalate; and / or the Nb source is selected from niobium pentoxide and / or niobium oxalate; and / or the lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium acetate and lithium oxalate; and / or the iron source is selected from any one or more of iron phosphate, ferric oxide, iron tetroxide, iron hydroxide, ferric lactate and ferrous oxalate; and / or the manganese source is selected from any one or more of manganese tetroxide, manganese carbonate, manganese dioxide and manganese trioxide; and / or the phosphorus source is selected from any one or more of phosphoric acid, iron phosphate, ammonium phosphate, diammonium hydrogen phosphate and ammonium dihydrogen phosphate.

[0045] Preferred carbon sources are a mixture of glucose and polyvinyl alcohol, and controlling their mass ratio within the above range helps to utilize their different carbonization temperatures, resulting in more uniform doping in the sheet-like composite lithium manganese iron phosphate material. Preferred Mg, V, and Nb sources are those mentioned above, as they are more easily dissolved in the liquid-phase molten salt medium, thus allowing for more uniform doping within the lithium manganese iron phosphate crystal structure and optimizing the lithium manganese iron phosphate lattice. Preferred iron, phosphorus, and manganese sources help improve their synergistic effect with other metal sources.

[0046] In one embodiment of this application, in step S1 above, the raw materials further include a solvent, preferably selected from any one or more of water, ethanol, methanol and glycerol, and / or the solid content of the slurry after grinding is 30% to 50%, and / or the abrasive particle size D50 during the grinding process is controlled to be 0.3 μm to 0.4 μm.

[0047] The preferred solvent is one or more of the types mentioned above, which helps to improve the uniform dispersion of raw materials during the grinding process, avoids material aggregation, and ensures the uniformity of the grinding process. The preferred solid content of the slurry after grinding is within the above range, which helps to improve the diffusion rate of lithium ions, thereby improving the energy density and performance of the composite lithium manganese iron phosphate material. By controlling the abrasive particle size during the grinding process within the above range, it helps to improve the diffusion coefficient of lithium ions, thus increasing the compaction density of the composite lithium manganese iron phosphate material while ensuring high capacity and high rate capability, thereby improving its charge-discharge performance and cycle stability.

[0048] In one embodiment of this application, in step S2 above, the protective atmosphere is a nitrogen atmosphere; and / or the sintering temperature is 700℃~750℃, and the sintering time is 6h~10h.

[0049] The above-mentioned optimized conditions are beneficial to improving the efficiency and effect of sintering, thereby improving the high crystallinity and stability of the structure and crystal form of the sheet-like composite lithium manganese iron phosphate material.

[0050] In one embodiment of this application, in step S3 above, the high-energy ball milling speed is 400 rpm to 800 rpm; the high-energy ball milling time is 3 h to 8 h; the mass ratio of black material to molten salt medium is 1:(3 to 5), and / or the melting temperature of the molten salt medium is below 600°C, preferably the melting temperature of the molten salt medium is 170°C to 600°C, preferably the molten salt medium is selected from at least two of LiCl, Li2SO4, LiClO4, LiBO2, LiAc, LiNO3, LiOH, Li2CO3 and LiBr; and / or the conductive additive is a sheet-like conductive agent, preferably the sheet-like conductive agent is selected from any one or more of sheet graphite, graphene and sheet MXene.

[0051] Controlling the rotational speed and time of high-energy ball milling within the above ranges helps improve its efficiency and effectiveness. Preferably, the melting temperature of the molten salt medium within the above range helps provide a liquid phase medium for high-energy ball milling, thereby promoting the transformation of raw nanoparticles into sheet-like particles. This solves the current technical problem of insufficient energy density caused by the inability to simultaneously improve compaction density and specific capacity due to the small particle size of lithium manganese iron phosphate materials. Preferably controlling the mass ratio of black material to molten salt medium within the above range is beneficial for improving the dispersion effect of the black material in the liquid-phase molten salt medium. The preferred conductive additives of the above types help improve the conductivity of the sheet-like composite lithium manganese iron phosphate materials.

[0052] In one embodiment of this application, in step S4 above, the temperature during the melting process is 400℃~600℃, and the holding time is 5h~10h.

[0053] By controlling the melting temperature and holding time within the above-mentioned ranges, it is helpful to improve the efficiency and effect of melting the activated mixture, thereby improving the electrochemical performance of the sheet-like composite lithium manganese iron phosphate material.

[0054] The beneficial technical effects of this application will be explained below with reference to specific embodiments and comparative examples.

[0055] Example 1

[0056] FeC2O4, MnC2O4, LiH2PO4, and Li2CO3 were weighed in a molar ratio of 0.4:0.578:1:0.008 and added to deionized water, then mixed thoroughly to obtain slurry (I). The solid content of slurry (I) was 40%. Glucose and polyvinyl alcohol 17-88 were weighed in a mass ratio of 8:1 as a carbon source, and the mass of the carbon source accounted for 12% of the total mass of the raw materials. 0.018 mol of Mg element (added in the form of MgO) was added according to the theoretical product. 0.002 mol of Nb (added as Nb2O5) and 0.002 mol of V (added as V2O5) were mixed and ground in a sand mill until the slurry had a D50 of 350 nm and a solid content of 40%. The ground slurry was then spray-dried in a spray dryer to obtain yellow material. The dried yellow material was then transferred to a box furnace and sintered at 700°C for 8 hours under a nitrogen atmosphere. After natural cooling to room temperature, black material was obtained.

[0057] Take 50g of the above-mentioned black material, add 200g of molten salt medium (LiCl:LiOH = 37:63 mol%) and 0.5g of conductive graphite for high-energy ball milling activation. The high-energy ball milling speed is 600 rpm and the high-energy ball milling time is 5h to obtain an activated mixture. Transfer the activated mixture to a box furnace and heat it to 500℃ for secondary growth under molten salt conditions. Then, allow it to cool naturally to below 50℃. Stir the powder in deionized water for 0.5h to remove the molten salt medium, and then place it in a vacuum drying oven for drying to obtain a sheet-like composite lithium manganese iron phosphate material with a thickness of 0.3μm and a sheet diameter of 1μm. The chemical formula of the composite lithium manganese iron phosphate core is LiFe. 0.4 Mn 0.578 Mg 0.018 Nb 0.002 V 0.002 The carbon coating in PO4 accounts for 1.8% of the mass of the sheet-like composite lithium manganese iron phosphate material.

[0058] Example 2

[0059] The difference from Example 1 is that the molten salt medium was replaced with LiOH and LiNO3. The melting points of LiOH and LiNO3 were measured to be 471.0℃ and 250.7℃, respectively, and the lowest eutectic point was 175.7℃. The composition was x(LiNO3) = 62 mol%, and finally a sheet-like composite lithium manganese iron phosphate material was obtained.

[0060] Example 3

[0061] The difference from Example 1 is that glucose is used as the carbon source, and finally a sheet-like composite lithium manganese iron phosphate material is obtained.

[0062] Example 4

[0063] The difference from Example 1 is that glucose and polyvinyl alcohol 17-88 were weighed at a mass ratio of 7:3 as carbon sources, and finally sheet-like composite lithium manganese iron phosphate material was obtained.

[0064] Example 5

[0065] The difference from Example 1 is that glucose and polyvinyl alcohol 17-88 were weighed at a mass ratio of 9:1 as carbon sources, and finally sheet-like composite lithium manganese iron phosphate material was obtained.

[0066] Example 6

[0067] The difference from Example 1 is that the solid content of the slurry after grinding is 50%, and finally a sheet-like composite lithium manganese iron phosphate material is obtained.

[0068] Example 7

[0069] The difference from Example 1 is that the mass of the black material remains the same, and the mass ratio of the black material to the molten salt medium is 1:5, finally obtaining a sheet-like composite lithium manganese iron phosphate material.

[0070] Example 8

[0071] The difference from Example 1 is that the mass of the black material remains the same, and the mass ratio of the black material to the molten salt medium is 1:2, finally obtaining a sheet-like composite lithium manganese iron phosphate material.

[0072] Example 9

[0073] The difference from Example 1 is that the activated mixture is transferred to a box furnace and heated to 600°C for secondary growth under molten salt conditions, finally obtaining a sheet-like composite lithium manganese iron phosphate material.

[0074] Example 10

[0075] The difference from Example 1 is that the activated mixture is transferred to a box furnace and heated to 700°C for secondary growth under molten salt conditions, finally obtaining a sheet-like composite lithium manganese iron phosphate material.

[0076] Example 11

[0077] The difference from Example 1 is that the high-energy ball milling speed is 800 rpm, and finally a sheet-like composite lithium manganese iron phosphate material is obtained.

[0078] Comparative Example 1

[0079] The difference from Example 1 is that commercially available nano-lithium manganese iron phosphate is used directly.

[0080] Comparative Example 2

[0081] The difference from Example 1 is that no molten salt medium was added, and the final product was a sheet-like composite lithium manganese iron phosphate material.

[0082] Comparative Example 3

[0083] The difference from Example 1 is that high-energy ball milling activation mixing was not performed, and the final product was a sheet-like composite lithium manganese iron phosphate material.

[0084] Comparative Example 4

[0085] The difference from Example 1 is that no Mg source, Nb source and V source were added, and the final product was a sheet-like composite lithium manganese iron phosphate material.

[0086] Comparative Example 5

[0087] The difference from Example 1 is that only Mg source and Nb source are added, and finally a sheet-like composite lithium manganese iron phosphate material is obtained.

[0088] Performance testing

[0089] Method for testing the lattice constant of sheet-like composite lithium manganese iron phosphate materials: obtained by XRD Rietveld refinement;

[0090] Test method for the sheet diameter and thickness of sheet-like composite lithium manganese iron phosphate material: Statistical analysis of particle size and thickness was performed using a Nano Measurer;

[0091] Methods for testing the mass content of Mg, Nb and V elements in sheet-like composite lithium manganese iron phosphate materials: ICP-OES analysis of doped elements;

[0092] Test method for carbon content in sheet-like composite lithium manganese iron phosphate material: Accurately weigh 0.2000g (accurate to 0.0001g) of sample into a ceramic crucible. After reading the weight with an analytical balance, add about 1.8g of tungsten flux and place it on the instrument crucible holder. Perform oxygen combustion test using a carbon-sulfur analyzer.

[0093] The lattice parameters, tap density, mass content of Mg, mass content of Nb and mass content of V of the sheet-like composite lithium manganese iron phosphate materials obtained in the above embodiments and comparative examples are listed in Table 1.

[0094] Table 1

[0095]

[0096]

[0097] Take the sheet-like composite lithium manganese iron phosphate materials from the above examples and comparative examples respectively, and mix them evenly with conductive carbon black (Super P) and binder (polyvinylidene fluoride) in a mass ratio of 80:15:5. Grind them thoroughly, add N-methylpyrrolidone, and then place them in a vacuum ball mill jar and ball mill at a speed of 600 r / min for 2 hours. Then, use a coating machine to coat the positive electrode slurry prepared above onto the surface of an aluminum foil current collector, and then bake and cure it at 110°C to form a positive electrode active material layer. Finally, use a roller press to cold press to the corresponding thickness to obtain a positive electrode sheet.

[0098] The above-mentioned positive electrode and Li sheet were used as the positive and negative electrodes of the lithium-ion battery, respectively. Celgard 2400 was used as the separator, and a 1 mol / L LiPF6 EC and EMC (v / v = 1:1) mixed solution was used as the electrolyte solution. The battery was assembled into a coin cell and tested. The specific test results are shown in Table 2.

[0099] Table 2

[0100]

[0101]

[0102] By combining the contents of Table 1 and Table 2, the following conclusions can be verified:

[0103] By comparing Examples 1, 3, 4 and 5, the necessity of using a composite carbon source in this example is demonstrated. Due to the difference in melting and boiling points, a single carbon source is prone to uneven carbon distribution during sintering. Using a composite carbon source makes it easier for the coated carbon to form a reasonable dispersion and contact surface on the surface of the lithium manganese iron phosphate material, thereby effectively improving the performance of the material.

[0104] By comparing Examples 1, 7 and 8, it can be seen that reducing the molten salt medium that provides liquid phase conditions will cause the particles to lack sufficient liquid phase conditions, resulting in insufficient secondary growth of particles during the synthesis process. Although reducing the molten salt medium in Example 8 will not cause a decrease in the electrical performance of its sheet-like composite lithium manganese iron phosphate material, it will lead to higher costs.

[0105] By comparing Examples 1, 9, 10 and 11, it can be seen that the temperature during the molten salt melting process and the material synthesis temperature both need to be reasonably designed. If the molten salt melting temperature is too high, the molten salt components will volatilize. If the material synthesis temperature is too high, the particles will grow excessively. That is, although the volatilized molten salt in Example 10 will not cause a decrease in the electrical performance of its sheet-like composite lithium manganese iron phosphate material, it will cause certain environmental pollution and an increase in energy consumption.

[0106] Comparative Example 3 did not use high-energy ball milling for activation. Due to the lack of high-energy extrusion and collision, the particles were not dense, and the tapped density was not improved.

[0107] The poor electrical performance of the sheet-like composite lithium manganese iron phosphate materials in Comparative Examples 4 and 5 fully demonstrates that doping modification is necessary for lithium manganese iron phosphate materials.

[0108] Figure 1 The phase diagram curve of LiOH-LiCl is shown. Figure 1 As can be seen from the data, the melting points of LiOH and LiCl were measured to be 471℃ and 605℃, respectively, and the lowest eutectic point of LiOH and LiCl was 294.5℃. The composition was x(LiCl) = 37 mol%.

[0109] Figure 2 The XRD pattern of the composite lithium iron manganese phosphate electrode material prepared in Example 1 is shown. Figure 2Comparison with the standard XRD pattern of lithium manganese phosphate confirms that Example 1 synthesized a pure-phase olivine-structured lithium iron manganese phosphate material. No impurity peaks were observed, and the peaks were relatively narrow with high peak intensity, indicating good crystallinity. Therefore, using mixed lithium salt as the molten salt medium did not affect the crystal synthesis, thus proving that this molten salt method can be used for electrode material synthesis.

[0110] Figure 3 The SEM image of the composite lithium iron manganese phosphate electrode material prepared in Example 1 is shown. It can be seen that the lithium iron manganese phosphate and graphite particles are mechanically fused through strong collision and extrusion, resulting in the particles agglomerating on the zirconium spheres. The detached particles are heated, and under the liquid phase conditions provided by molten salt, the micro-particles continue to grow, eventually forming a sheet.

[0111] Figure 4 The SEM image of lithium manganese iron phosphate purchased from the market shows that the primary particles are very small, which makes it difficult to increase the compaction density of the material and also results in a high viscosity of the slurry during the mixing process, making coating difficult.

[0112] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0113] In the sheet-like composite lithium manganese iron phosphate material of this application, by doping the lithium manganese iron phosphate structure with Mg, V, and Nb elements, the advantages of each doped metal ion can be combined, thereby improving the crystal structure, conductivity, and cycle performance of the sheet-like composite lithium manganese iron phosphate material. Specifically, due to Mg... 2+ Ionic radius smaller than Mn 2+ The MO bond length in the doped MO6 octahedron becomes shorter, while the Li-O bond length in the LiO6 octahedron becomes longer. This extension of the Li-O bonds results in the Li... + The diffusion channels are wider, Li + Easier migration leads to increased carrier density in sheet-like composite lithium manganese iron phosphate materials, which in turn benefits the electrochemical performance of multi-component olivine-structured cathode materials. The introduction of multivalent metal ions V... 5+ and Nb 5+ Ion defects were generated, and mixed valence states favored Li + Intercalation / deintercalation within the material. This involves altering the crystal structure of lithium manganese iron phosphate (LFP) through ion doping, creating defects in the original LFP lattice, thus enabling Li… +The expansion of diffusion channels and the increase in carrier density of the sheet-like composite lithium manganese iron phosphate material can improve its electrochemical performance to some extent. Therefore, by doping with Mg, V, and Nb elements, the lithium manganese iron phosphate nanoparticles are restructured into a sheet-like morphology, resulting in a sheet diameter of 0.5 μm to 2 μm. This improves the powder packing density of the sheet-like composite lithium manganese iron phosphate material compared to the original morphology, thereby increasing its energy density. Furthermore, the carbon coating layer significantly enhances the energy density, discharge specific capacity, rate performance, and cycle stability of the sheet-like composite lithium manganese iron phosphate material.

[0114] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a sheet-like composite lithium manganese iron phosphate material, characterized in that, The sheet-like composite lithium manganese iron phosphate material comprises a composite lithium manganese iron phosphate core and a carbon coating layer covering the surface of the composite lithium manganese iron phosphate core, wherein the chemical formula of the composite lithium manganese iron phosphate core is LiFe. x Mn y A z PO4, where 0.2≤x≤0.4, 0.57≤y≤0.78, x+y+z=1; A represents Mg, V, and Nb. The sheet-like composite lithium manganese iron phosphate material has a sheet diameter of 0.5μm~2μm and a tap density of 0.9g / cc~1.2g / cc. The preparation method includes: Step S1: Mix raw materials including lithium source, iron source, manganese source, phosphorus source, carbon source and doping source and grind them to obtain a ground slurry; Step S2: Spray the ground slurry to obtain yellow material, and sinter the yellow material in a protective atmosphere to obtain black material; Step S3: The raw materials including the black material, molten salt medium and conductive additive are activated by high-energy ball milling to obtain an activated mixture; Step S4: After the activated mixture is sequentially melted, filtered, washed and dried, the sheet-like composite lithium manganese iron phosphate material is obtained. The doping sources include Mg sources, V sources, and Nb sources; In step S1, the carbon source is a mixture of glucose and polyvinyl alcohol, and the mass ratio of glucose to polyvinyl alcohol is (7~9):(3~1). The mass ratio of the black material to the molten salt medium is 1:(3~5), and the melting temperature of the molten salt medium is 170℃~600℃.

2. The preparation method according to claim 1, characterized in that, In step S1 The Mg source is selected from any one or more of magnesium oxide, magnesium carbonate, magnesium acetate, magnesium lactate, magnesium dihydrogen phosphate, and magnesium hydroxide. And / or the V source is selected from any one or more of vanadium pentoxide, ammonium metavanadate, vanadium carbide and vanadium oxalate; And / or the Nb source is niobium pentoxide and / or niobium oxalate; And / or the lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium acetate, and lithium oxalate; And / or the iron source is selected from any one or more of ferric phosphate, ferric oxide, ferric oxide, ferric hydroxide, ferric lactate and ferrous oxalate; And / or the manganese source is selected from any one or more of manganese tetroxide, manganese carbonate, manganese dioxide, and manganese trioxide; And / or the phosphorus source is selected from any one or more of phosphoric acid, ferric phosphate, ammonium phosphate, diammonium hydrogen phosphate and ammonium dihydrogen phosphate.

3. The preparation method according to claim 2, characterized in that, In step S1, the polyvinyl alcohol is 88 series polyvinyl alcohol.

4. The preparation method according to claim 1 or 2, characterized in that, In step S1, the raw materials further include a solvent, and / or the solid content of the ground slurry is 30%~50%, and / or the abrasive particle size D50 during the grinding process is controlled to be 0.3μm~0.4μm.

5. The preparation method according to claim 4, characterized in that, In step S1, the solvent is selected from any one or more of water, ethanol, methanol, and glycerol.

6. The preparation method according to claim 1 or 2, characterized in that, In step S2, the protective atmosphere is a nitrogen atmosphere; and / or the sintering temperature is 700℃~750℃, and the sintering time is 6h~10h.

7. The preparation method according to claim 1 or 2, characterized in that, In step S3, the rotation speed of the high-energy ball mill is 400 rpm to 800 rpm; the high-energy ball milling time is 3 h to 8 h. And / or the conductive additive is a sheet-like conductive agent.

8. The preparation method according to claim 7, characterized in that, In step S3, the molten salt medium is selected from at least two of LiCl, Li2SO4, LiClO4, LiBO2, LiAc, LiNO3, LiOH, Li2CO3 and LiBr.

9. The preparation method according to claim 7, characterized in that, In step S3, the sheet-like conductive agent is selected from any one or more of sheet-like graphite, graphene, and sheet-like MXene.

10. The preparation method according to claim 1 or 2, characterized in that, In step S4, the temperature during the melting process is 400℃~600℃, and the holding time is 5h~10h.

11. A sheet-like composite lithium manganese iron phosphate material, characterized in that, The material was prepared using the method described in any one of claims 1 to 10.

12. The sheet-like composite lithium manganese iron phosphate material according to claim 11, characterized in that, The lattice parameters of the sheet-like composite lithium manganese iron phosphate material are: 9 Å ≤ a ≤ 11 Å, 5 Å ≤ b ≤ 7 Å, and 4 Å ≤ c ≤ 6 Å.

13. The sheet-like composite lithium manganese iron phosphate material according to claim 11, characterized in that, The mass content of Mg is 2000ppm~3000ppm, the mass content of V is 100ppm~1000ppm, and the mass content of Nb is 1000ppm~2000ppm.

14. The sheet-like composite lithium manganese iron phosphate material according to claim 11, characterized in that, The thickness of the sheet-like composite lithium manganese iron phosphate material is 0.2 μm to 0.5 μm; and / or, the mass of the carbon coating layer is 1.5 to 2% of the mass of the sheet-like composite lithium manganese iron phosphate material.

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